Depends if your running 32 bit XP then I would say 2gb of 5400 would do just fine and overclock that.
If your running 64 bit vista 3GB+ is always best for performance. I never seen my XP 32 Bit partition ever use more then 2GBs unless I overloaded the memory on purpose.
Also if you tweak both 32 and 64 Vista then you can actually hault its normal ram under 800mb in use....I never see it pass 2gb unless a game can use the cache (such as GTA4)