well whats your budget for the rest of the system? If you can fit in a better intel processor like an LGA 1366 based i7 then do it, if not a Phenom II X4 955 or i5 760 will do just as well(better than the 1090T) especially if you OC them.
The answer seems to be not much when it counts (eg minimum frame rates). Findings seem to show that when you can generate a high enough frame rate to "bottleneck" x8-x8, your framerate is so high the lost frames don't matter.
This article shows a test using a 4.0GHz 920 (to minimize cpu issues) and 2x480s (to provide "ample" gpu power). The mobo allowed choice of "bandwidth", ie, x16-x16, x8-x8, x16-x4. The results are interesting, but no one that I've seen has done the direct 920 vs 750, 2x460 , 1920 resolution comparison.
For i5 750, you have to get a P55 (LGA1156) board, which supports SLI/CF in 8x/8x mode. With the X58 (LGA1366) boards you can do a 16x/16x SLI/CF. But there is not much of a performance hit in there. So, no problem.