As long as the RAM is compatible
That's the point - but quite often "compatible" memory is difficult to obtain.
(which the fact that the PC can still boot would support)
Often a system will boot but show instability signs later. I don't want to belabor this, but wanted @chevyavalanche04 to be aware that RAM upgrades may not be as simple as "plug in new RAM and go." Quite often they are - quite often they are not. If problems occur, all he has to do is to remove the new RAM and ask for help.
As far as performance goes, >1GB on XP can be barely noticeable depending on use of computer; for Vista, 2 GB is essential and will be noticeable from boot time (reduced) and throughout the user experience.