My system is in my sig. I would like to buy a 7800GTX soon. I know I'll need to get a PCI-e motherboard, but what can I re-use? What MB would you suggest? Can I use My CPU?
I'd grab a new Nforce4 based motherboard, but other than that, you don't really need to upgrade anything else. The PSU (might have to get a molex->pci-e adapter) is more than fine, the RAM is fine (unless you're really serious about Battlefield 2 in which case another gig would help), and the HDs should be fine.
If you don't think that you'll ever consider going SLI, you should be able to save some money by going with a non-SLI motherboard.
The only other upgrade that I would consider is the CPU. It might be worth selling the CPU on the forums here, get $100 for it, and grabbing a quicker proc. The reason I suggest this is because it will be holding the 7800gtx back a little. This 3700 is a good option.