CPU surley plays a role! especially with Crossfire. But a Q9550 is a realy good CPU, even if it is old! you're now on 3.4ghz with 1.04V and standard CPU cooler thats nice, but H80 isnt that powerfull. Normal Big size Air Cooler are better
Tests here... See for yourself. If you like it buy it, but there are other options. i Think you should be able to go to 4ghz :) cause Q9550 allows up to 1.3625V ... well wouldn't force 4ghz or 4.2ghz with 1.36V but if 3.7 or 3.6 runs with 1.25V should be enough.
About GPU..... HD6990 or 2x6970... first said. you don't realy have a performance loss if you use a 6970 in a 8x PCI-E Slot!
Tests here (sorry in german ;) ) 16x16x is a HD6990 witch clockspeed of a 6970.
So 2x 6970 would be More Powerfull, 1. They're running faster than a single 6990, and 2nd you can OC them more. mine works at 930 instead of 880.
Important side Note! 3 Displays wont run on a Single GTX580 (nor on a gtx590 ?) without Matrox thing. but they would on a hd6990 or 6970.
btw, if your low on cash, buy two OLD version! (check twice beforhand!) HD6950, do a shadder mod (or bios flash) and OC them to 880mhz.
greets Haldi