another set of 2x1GB ram sticks, assuming you have the open slots on your mobo (just be sure to get a similar pair to the one you have in your system)
as far as the vid card goes, to stay within budget, I'd say a gtx460 if you're an nvidia person, or a radeon 5830 if you're ATI. Can't go wrong either way IMO.
The ram will run you about $50, the cards are ~$200. Those would be the logical upgrades for price range. You can always go bigger on the ram since you're going 64bit if you want to go to 6gb instead of 4. That just might take the vid card down a step, so its up to you.