Well with your budget it should be no problem going for i7 but the MOBO is more expensive for it which means you might have to go lower on the GPU..... Also i'm not too good in putting an intel build together as i think AMD is better bang for the buck.
8x/8x loses at most 2% of 16x/16x performance of the very fastest GPUs, less on midrange GPUs like the 460 and 6850. This translates to 98 fps instead of 100 fps. or 49 instead of 50 on a GTX 580, HD 5970, or the like.
Its likely to be 1 FPS or less to use 8x/8x instead of 16x/16x for crossfire