I have to agree with Kin, plus if your PC is old enough that is has less than a gig in there then the mobo may not even take 4 gig. XP with 2 gig and a decent CPU should perform very well indeed. Less so Vista...
Crucial test your system to see what it can take.
It"s only about 3 years old - it"s more to do with that I got it very cheap (at least partly to do with the lack of RAM). And that"s the easiest thing to upgrade, so as the rest of it was fine I couldn"t turn the deal down, it was too much of a bargain.