I'd go with an AMD chip - and to that end you'd need:
Either an Asus or a Chaintech motherboard (I've gone for the Chaintech and loved it, but it has frills and is slightly less powerful than Asuss board - it's a choice thats up to you).
The new AMD Barton chip just got launched which may well drive current CPU prices down somewhat, give that a little time to take effect - although the current price on a 2100+ XP is pretty much nothing and they run fast enough, so again it's up to you.
A new case is always a good idea purely for cooling purposes - I intend to follow my own advice very soon

Memory? DDR... the faster the better but again it's down to you.
Really it's all about what you can afford and what you want, then balance that out. If you put an idea together and it comes out to steep then drop the memory speed slightly, or go for a slightly slower processor. Whatever works

You've got a good graphics card so the faster the other components the better to do that justice.