Could you please post the cpu temp. from speed fan and what kind of cpu you're using...
Are you using a video card or onboard video?
When deciding on what psu to purchase, you first have to determine what your amperage (amps) needs are for each voltage rail. The cpu uses amperage from the 12 volt rail... CD/DVD drive moter, hard drive moter, and fan moters all run off the 12 volt rail as well. If you're using a video card, that will also run off the 12 volt rail. The logic circuit on the drives will use the 5 volt rail... The point is that you need to look at the power rating of the components installed in your computer. For instance, the CD/DVD drive in my computer uses 1.5 amps from the 12 volt rail for the moter, and 1.3 amps from the 5 volt rail for the logic circuits (as stated on the drive). Find out how many amps each component in the computer draws and on what rail. Once you've got all that info. you do a little math. For example, my cd/dvd drive uses 1.5 amps/12 volt, and 1.3 amps/5 volt. Multiply the amps * the volts to determine the total wattage used. 1.5 x 12 = 18 watts. 1.3 x 5 = 6.5 watts. Add those together and you get the total wattage for the drive (24.5 watts). Do that for all the components on the system and add the wattage of each component together and walla, you've got your total wattage needed. (it's not easy to determine the total wattage of the motherboard itself so you may want to use that online calculator to determine its wattage)
Just be sure that whatever psu you purchase offers enough amperage on each rail for your components. If the 12 volt rail for example, offered 15 amps max, and total amperage used by the components was over 15 A, it wouldn't work (well, it might, but let's not push it)...It's also advised that you go a little over the required amperage needed, so you don't max out a rail at any given time. Also, Once you've got the total wattage added up for your system, add 15 - 20 % to leave room for expanding and/or anything you may have missed in calculating.
Any given power supply will not supply max. amperage on all rails simultaneously, but an average user is not likely to draw the maximum power possible for each component anyway, so you might wonder why adding 15 - 20 % is necessary; like I said, you never know when you may need a lot of power and you'll want room for expansion and/or miscalculations. And be sure to get a psu that uses a 24-pin main connector (not a 20-pin) otherwise, if you decide to use a PCIexpress card, you'll have to use an adapter that connects the 4-pin molex connector to the 6-pin connector on the PCIexpress card. The 24-pin main connector on a psu corrects the lack of power that the 20-pin connector can deliver...
There are also other specifications you may want to look for in a supply, such as PFC (power factor correction) and over-voltage protection, and cooling aside from many other possible specs.... Cheap supplies with bad cooling heat up and don't deliver the rated power output. The hotter they get the less they can deliver power.
If you're unable to figure out how much current a component draws, just look at the rated power draw of the components, in "watts". For instance, a "100 watt" PCIexpress card will run off the 12 volt rail, which means, to find out the amperage draw for the card just divide the watts by the voltage. 100 watts divided by 12 volts is approx. 8.3 amps...
Note: The online calculator you used suggested a 278 watt supply, so you figure just get a 300 watt supply right? Not exactly, going out and getting any supply just because it offers 300 watts doesn't mean its amperage output per rail is sufficient for your particular needs...Be sure to look at the amps available per rail.
Don't pay attention to the +5v SB on the psu when determining how much power it has....Disregard the -5 and -12 voltages on the psu unless you plan on using legacy (old) hardware that runs off negative voltage... They are not counted in the max. power out put, if no devices are using them..
