Your CPU will bottleneck anything stronger than a HD4850. With Bulldozer and Sandy Bridge due out soon, investing too much into a new CPU would be be a waste. If you PLAN on a future rebuild for a new mobo, CPU, and RAM, get a GTX460 now. Yes, it will be bottlenecked for now, but you'll be able to move it into the new build later.
Make sure your PSU can handle it. Considering future upgrades (keeping the single GTX460), you'll want a quality 500W PSU. It should have full range active PFC (no little voltage switch) and be 80+ certified. Antec, Seasonic, Corsair, and Enermax are among the better brands.