Your budget will definitely be the restricting factor.
According to Asus, you mobo can handle 45nm processors as well as quads up through the Q9650. Basically, you could drop in anything you could afford. The cheapest quad NewEgg has is this:
Newegg.com - Intel Core 2 Quad Q8200 2.33GHz 4MB L2 Cache LGA 775 95W Quad-Core Processor - Processors - Desktops
This one does not have virtualization, so it would not be able to run XP mode should you go with Win7. The Q9400 does, but it is $40 more.
If you do decide to go with Win7 (or you are currently using 64-bit Vista), you may consider bumping your RAM to 4GB. DDR is not cheap anymore, but neither is DDR2 or DDR3 right now. On both the RAM and a quad core you may be able to catch some good deal on ebay.
As for a graphics card, Muffin is right about checking your PSU. BFG has a great 550W PSu for $50:
Newegg.com - BFG Tech GS-550 550W ATX12V V2.2 / EPS12V V2.8 SLI Ready CrossFire Ready Power Supply - Power Supplies
The 4870 is an awesome card, by the way.