I can't give you any links, because I am speaking of my own experience. I have an E6400 and a 4850 and it is NEVER used at its full speed. I think only Crysis has taken it to 100%. When I benchmark with gpu-z, which shows you what percentage of your card is used, it is around 50-80% all the time, while the cpu usage approaches 100% (for one core or two cores, depending on the game; I still play some games that have minimal or no multicore support like TF2 and NW2).
I told you that I have oc'ed my E6400 to 2.56. I do believe however that if you manage to OC it to 3.2, then a 4850 or a 4870 should not be a bottleneck, especially at your resolution.
I play at 1440x900 and with some games I get low fps. Crysis with a mix of mid-high settings, I get 25-40 fps (lower in a really heavy battle). In TF2 depending on the situation I get from 35 to 140. I used to get 20 before overclocking.
Anyway, I am not saying you should buy a 4850 and nothing better (I am not that pleased with my card; ever since I got it it idles at around 70 degrees). DX11 comes out, so it would be wise to go with a 5xxx, especially if you plan on getting Windows 7. I am just saying that with your cpu (and mine, remember that), there is no way that the graphics card is going to be used at its full capacity. That's why I suggested the 5770. If it's any good, it sounds ideal for your system.
In any case, I hope you enjoy your purchase.