ATI 4770/4850/4870 peformance comparison on a smaller montior

I've been interested in upgrading my video card, but I'm not really sure where to go. I'm currently on a 17" LCD at 1440x900, running a 3850 512MB.

The 4770 is really attractive to me due to its low price, smaller size, and low power draw. However, the extra power given by the more expensive cards is appealing. I see people recommend cards like the 4830/4770 for the resolution I play at, usually stating that there isn't much performance increase until you up the resolution.

Since I've never seen any FPS charts for these cards at this resolution, maybe you guys can help me out. How would the performance of a game like Left 4 Dead vary between these cards? If there is no real discernible in a game like this, would there be a large difference in newer games? Would I be better off just waiting until I come across a game my 3850 can't play before upgrading?

Also: Yes.
As a rough idea look at the 1280x1024 resoloution in benchmarks, it is slightly off, but only by a small amount, enough for 'ball park' comparaisons.

@ gwarm01: Yes, the HD4870 is faster.
Some might question the value of such a fast card at such a relatively low resoloution but for gaming it is always better for the system to have a bias towards GPU rather than CPU power.
If your system has a fairly fast CPU (C2D E6XXX or Q6600) or similar AMD unit then go for the faster card when or if you choose to upgrade. If the CPU is slower then go for the HD4770 or Nvidia rival.