For hardcore gaming, there are 4 important things:
- Graphics card
CPU: I've always been an Intel user, but I hear AMD is better for gaming. Get AMD64 if you can afford it.
Mobo: Not a clue. Maybe one with PCI Express, and good RAM amounts.
RAM: at least
1024MB, preferably DDR or such like.
Graphics card: You really should go nVidia - they're better than ATI just now. Have a look at the 6800GT or 6800 Ultra, but make sure the card you choose is compatible with the games you want to play (Battlefield 2 especially).
You could also, I suppose, overclock some of that kit to make it go even faster. But you need to have some nice cooling gear to make that work effectively.
HTH. Let me know if you have any more questions.