Well with 400 dollars you can go extreme overkill, especially with that resolution. I'm all for going all out, but I think a 460 1GB for 230 dollars would be fine for you. A 470 would be about 315 dollars and I think that would be too much, then again it will last longer.
The best option would be a GTX 460 1GB.Its an excellent card for that resolution and when overclocked, it matches the performance of a HD 5850.You can also add another one in the future and two of them in SLI will easily outperform a GTX 480.
I don't get this overclocking and compare. A HD 5850 is better then the GTX 460 when going stock. If your going to overclock the GTX 460 to match the HD 5850 then why not overclock the HD 5850 to beat the overclocked GTX 460.
And to up that, two HD 5850 in CrossFireX beat the GTX 460 in SLI. You have ot compare apples to apples, so compare cards at stock or BOTH overclocked. You can't give a fair comparison if your Overclocking one and comapring it to the stock of another.