Usually adding RAM is a doddle but there are a few things to watch out for.
Make sure that the memory you install is recognised by the BIOS before booting into an OS, I had huge problems once when I booted into my OS's to run diagnostics on a memory stick which wasn't being reognised by the BIOS. In short the OSs were using the memory despite the BIOS not recognising it. It sent them doolally.
If your BIOS allows different sizes of memory sticks then you can often use two different speeds of memory, but the system wil run at the speed of the slowest so better to buy the cheaper, slower sticks which match your present one. Some people reckon that having 2 matching sticks i.e. either buying another 256meg stick or two 512 meg sticks improves system performance. Can't say I've ever tried it though.