Faster ram is irrelevant if your system cannot handle it. Buy the maximum amount of Ram your system will need. For XP the sweet spot is around 1gb. Unless your an avid gamer, you won't need more. It is best to buy in twin pairs. for 1GB - get 2 sticks of 512mb. That way you can run in dual channel mode and your system will run slightly faster.
For 2 gb , buy 2 x 1gb sticks.
If you buy fast ram and your motherboard cannot handle it, it either will not work or it will run at the maximum speed your board can run it at - and you've wasted money. RTFM to determine what brands, types, your system will take. IF no manual - check with the mobo manufactuer website.