The difference between x8,x8 and x16,x16 is less than 3%. What I would do personally is run a pair of 6970's because you will get better framerates from them most of the time and they are cheaper.
Actually even HD 6950's in CF give the GTX 580's a run for their money
However if you are set on the GTX 580's well I guess you could go for them. Truthfully there are very few situations where that extra vram helps even if you are recording... Another option for the same price as the GTX 580's is tri-fire a 6990 with a 6970. Then you are really gonna kick the 2 580's ass.
Introduction - AMD Radeon 6990+6970 CrossFireX / "TriFire" Review | [H]ard|OCP
P.S no video capture card?