Well, if my experience going from a 260GTX to a 5850 is anything to go by, you'll see some improvement. Did you also add more RAM, though?
My old system:
Core 2 Duo 2.33, 4GB RAM (only 3 available, since I ran XP 32 bit), GTX260, running at 1900x1200 resolution
New system:
i7 2.80 quad core, 8GB ram, 5850, windows 7 64 bit, 1900x1200 resolution
VAST improvement, as in, before I ran with main window settings on medium, 4 side windows on lowest, throttled non-active window FPS to 10 and still lagged a bit when swapping windows or, god forbid, going to Dalaran. If I turned on mouse broadcasting my system would literally overheat and bluescreen inside of 30 seconds. New system I don't have to throttle if I don't want to, and just for fun I ran all 5 windows at maximum settings and was still getting about 30ish FPS in each.
I'll still run everything throttled, since I can't really see those windows very well, and probably reduce the settings just to avoid heat issues, but the difference was *huge* I'm still thinking of another couple of upgrades: going to 16GB RAM and adding a SSD for the OS and gaming partitions.
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