I wouldn't go for the XP2500 ... what's your price range ?!?!
You may as well get 64bit ... may aswell be future proof ... for up and coming applications
I would get PCI express for the same factor (future proof) lol i have AGP atm ... stick with that i would say for now ... unless you have enough money that is ... you won't see THAT much difference i don't think unless you got a GFX x 2 ...
Give us some expenditure costs that might help abit

lol
Things i would DEFFO upgrade to ...
RAM

BIG improvement lol
Sound card - unless you really want to ?!?! Motherboard one should be fine ... unless your really into like sound quality of games etc.
HDD - yes i would say

GFX - Yes deffo if you can afford it

lol