this sort of upgrade is really simple. because you only have 2 slots, you have to make sure you've made up your mind on how much ram you want. if you just run basic applications in windows, 512mb should be enough (2x 256 or 1x 512) but if you play games, i suggest you go for 1 gig (1x 1gig or 2x 512) about that DDR 333, DDR 400 etc. those are the frequencies of the ram. the higher the frequency, the higher the bandwidth. so get RAM modules that are DDR 400.
Also, try to keep the modules the same brands if you're planning to use 2. I'm not saying it won't work, but for stability's sake, try to keep it the same brand.
hope this helps