The only need to have the OS and Storage Data on a separate drive would be if you run into a capacity limitation problem as BC stated in its size, however if you are using this system for say video editing in which you are working with large files and making changes to them in which there are excessive amounts of read/write instructions, by having the work area that the video editing is occurring at on a separate drive, the SSD as the OS drive wouldnt lag as a result of a communications bottleneck with that specific drive.
A few years ago I had a system with just a 90GB SSD in which the OS ate up 30GB of it for Windows 7 fully patched and of the 60GB remaining space 24GB of that was used by a video game leaving me with around 36GB storage area. I use a program called FRAPS to capture and record online game events with the group of people I play with, and FRAPS makes for very large video files which then need to be encoded down to a far smaller size than a raw video dump to disk. So I would have 18GB raw video dump files and use virtualdub64 and convert them to like 700MB AVI files and delete the original 18GB raw video dump file. But when this was happening the hard drive was extremely busy with read/write conditions as its processing this very large video file to make it into a nice 700MB AVI file to share on youtube with the group of friends.
When the system was encoding the video file the SSD while fast, was busy and so everything else you wanted to do during that time lagged with opening games and programs.
By adding a separate drive for this video editing/converting could occur on, the lag I had with wanting to play games while this is happening was gone as for the much older 164.7GB SATA HDD while a slower SATA 1.5 drive, was handling all the conversion process read/writes and my SSD was pretty much running idle ready to jump at the occasion to retrieve information quickly.
Lastly, for my config I changed my swap file to be targeted on the HDD vs the SSD because the video conversion process even though I have 4GB of DDR2 800Mhz RAM, was swap intensive and with default of C: for the swap space which was my SSD, so I let the HDD handle it all. The video games while they too use the swap file didnt seem to have a bad performance hit using the swap space on the HDD that is busy with the video conversion.
If your not video editing and the drive is large enough to allow for the OS and Storage to work in harmony then the single drive will be just fine. Just make sure you dont get too small of a SSD as for I ran into a problem with an older system I have that I installed a 40GB SSD with Windows 7 on it and Windows 7 use to consume only 18GB space clean install, but after all the updates it grows. Currently Windows 7 is 27.3GB so it grew about 9.3GB in size since installed clean from Disc about 2 years ago. On the 40GB SSD while storage space was already kind of tight I made the mistake of installing programs in the free area when I was at 18GB for OS size, 1GB for user profiles, and had about 4GB free space. As Windows 7 swelled in size as patches forced it to consume more space I soon found that the SSD was out of free space and Windows 7 updates were failing. I then had to uninstall and reinstall programs to the other HDD in the system and get them off of the SSD so that Windows 7 could continue to grow in size with patches from Microsoft as well as perform some profile cleanup to get rid of unnecessary files that were sitting in the user profile space which was also on the SSD.
With SSD's cheap these days, I wouldnt buy any that were lesser than 120GB in which the last SSD I got was 240GB for $125 for a Crucial M500 240GB SSD
