XP CAN handle a maximum of 4, but you may want to confirm your motherboard can handle it.
Thanks blueeyesm for the advice. I'll be using Vista 64-bit so I'll be ok with the 4Gb limit of 32-bit. I'll be using an Asus Striker Extreme - I havent 100% checked yet but I'm pretty sure it should work.
They will fit fine on StrikerExtreme, however if u still havent bough the Striker, then go for ASUS P5N32-E SLI PLUS, its alot cheaper and has the same features (the reason that Striker is expensive is that it has some extra features like LCD poster and also ASUS has called it a gaming motherboard which is mainly advertising)