While its not impossible to have a bad or flawed CPU, I question how the "store" determined it to be the CPU. Does this store have a good rep for selling trouble free products and are their employees knowledgeable in what they are doing?
The motherboard should be fine as for Gigabyte makes solid motherboards or good quality. With the power supply already swapped out and same symptoms, I'd be trying different RAM or a different Video Card. I have seen both cause a system to reboot when they act up with the video card being more common for a hard reboot where the screen just goes black and then a beep and reboot. Generally with memory issues you see a quick flash of a memory address failure etc or it gives you a blue screen of death memory dump message.
If you have a video card or extra RAM handy of similar spec's, you can try swapping out one or the other.
Generally RAM can be 99.9% proven to be good thru testing it with Memtest86. If it runs memtest86 for like 3 full tests and no red error messages then the memory should be good. As far as video card goes it can act up randomly and cause problems, however performing a stress test on the video card might cause it to trigger a reboot if the video card is flawed.
In regards to your questions:
SSD's are as easy to use as HDD's. They install the same. But they are way faster when you have a good quality SSD.
Pros and Cons to SSD's .....
Pros = [ Fast, can be bumped, vibrated next to a woofer, or dropped and will still function due to no moving parts inside, and Lower Power Demand from PSU ]
Cons = [ Expensive for large capacity, Data Recovery options are very limited vs HDD's ( more likely for a total data loss if no backup is implemented ), While a HDD can work well for Heavily Read/Write intensive applications ... SSD's can burn out the cells that hold the data and some will shrink in capacity as cells die and others can just completely die, you want to use Windows 7 or newer OS for best performance with them as for some of the advanced features to keep the SSD's healthy are not available for older OS's such as XP as well as I have heard that some Linux OS are unable to condition the drives to avoid cells from being overworked and garbage control cleanup of old data etc. * Some manufacturers of SSD's add a feature to them that monitors the write traffic to the drive and so if you are hitting the drive heavily and exceed what the manufacturer determined as the value of TB in writes for a given period of time or given limit ... the drive will throttle itself back to slow down performance to last longer ... To me this is a (sneaky) means by which the manufacturer can just about guarantee that the drive will last the 3 years of the warranty without being burned out and people demanding free replacements in a never ending cycle of free replacements after frequent overuse which would cause a loss to the manufacturer.]
Life span all depends on usage mainly and how its treated. I ran a utility on a SSD once that said that under the current usage of the SSD that it would last 46 years. But this was also because I had the SSD installed as C: and a HDD installed as D: and the SSD was used mainly for booting the OS and launching games, while the HDD was used for storing personal data and the swap file which is constantly causing read/write conditions. So by keeping the read/write of the swap file off of the SSD, this utility gave me a life expectancy of 46 years where the SSD drive was used mainly for read-only operations which is where the speed is in performance of SSD's. The writes and overwrites are what wear the cells in SSD's so if you wanted to optimize the life of a SSD, you can use a HDD for the heavy hitting read/write processes and use the SSD mainly for read-only types of operations. In most laptops you dont have this option, as well as for the most part you might as well just run the SSD hard because in say 4 or 5 years when its ready to die you will want to have a larger capacity anyways, and so you would get the most of it vs retiring a lightly worked SSD in 5 years etc.
As far as is it better to have one... I have to say it is better to have one if the application is best suited for a SSD. That is if you are video editing and creating massively large video files and then editing them and then encoding them to a different format, I would not suggest a SSD for this application if you want the SSD to last. However if your a business and dont care about burning out 1 SSD per year and time is money and having a video ready in 10 minutes instead of 20 minutes is important, then have at it with runing a SSD hard because chances are the time saved in processing the large files faster if you added up all the saved time, the saved time = saved money and so the SSD even though burnt out in say 1 year still saved your company a load of money. However if your a home user and want it to last and dont mind a video processing task taking 20 minutes instead of 10 minutes, you can save your SSD from being overworked.
* I have systems with both SSD and HDD's in them and I mainly use the SSD's for the quick boot and games and when I process large videos of gaming events I do all of this on a SATA II 500GB HDD to work the mechanical hard drive that is better suited for this type of task and lasting in the long run. I plan on getting about 7 or 8 years out of my SSD's instead of 3 to 5, that is as long as the capacity of the drive allows for installation of an OS and game or two and as long as nothing faster and cheaper came along which would make the current SSD a waste of time to operate when I can have faster and at little cost ... This may be wishfull thinking
But everyone I know who has a SSD are happy with them, however you will want to buy one that doesnt confine you to a tight box of space to work in. That is, there are SSD's out there of say 30GB that can be bought cheap for $29.99 on a close out deal, and while they are FAST, (faster than HDD's ) but not the Fastest SSD out there!!, you may soon find yourself with not enough space to do much with them depending on application. I own a bunch of SSD's and have some smaller capacity drives and the problem with them is that its tight to install the Windows 7 OS and any of the better / larger games that would benefit from the SSD performance. So I would suggest not going cheap, and not going expensive, but buying an in between price tagged middle capacity size SSD. Such as I bought a 240GB M500 Crucial as seen at link here and it has plenty of room for OS and games and is a really solid and fast SSD
http://www.newegg.com/Product/Product.aspx?Item=N82E16820148694Lastly, having a larger capacity SSD has the benefit of more flash memory cells in the pool of available cells to write to with larger capacity and so with a larger capacity drive, the frequency at which a cell is re-written to is less frequent than that of a smaller drive which if the drive was say 90% full, there are 90% of the cells at a constant fixed state of 0's and 1's and the remaining 10% of the SSD where swap space is happening is frantically written and over-written over and over again and so your overworking a small 10% area of the SSD with the other 90% of the drive not as exercised and so your prone to failure much sooner than later mathematically with a smaller SSD than a larger SSD, but also if you have a larger SSD and its jam packed and you only have 10% free space, then your still stuck in this pickle of overworking 10% area of the drive and this will lead to a failure sooner than later.