Compatibility was an issue for me when I first upgraded my old computer, about 10 years ago. A lot of things have changed. A good way to find out if a component is compatible with another is to go ask in forums such as these, or while shopping in places such as Newegg, read the reviews. Most people tend to write what they used with their motherboard, what Ram and video card worked the best etc. Like many people here, I built my own system, and have not had a single hardware or software issue. Also I have not had a single blue screen on Windows XP. This is a great improvement for someone who used to re-install Win 95 every other week.
Also, since manufacturers such as dell, Compaq etc, tend to use propriety parts, you might want to try to salvage whatever you can out of the system, and put it into a case of your own, though a little more costly, it is likely that you will have less compatibility issues with size and PSU.