It's very unlikely that any single card will be BN'd to any significant degree by any decent quad at 3.8GHz as long as your gfx settings are reasonably high.
Two 560's though, I'd want to at least have an i5, personally. Not because the BN is going to be terrible with your current CPU but just cause if you're gonna spend that kinda money on GPU's, you really don't want to be feeling like you're being held back at all by CPU and platform ... and you would be, at least in certain scenarios, with that CPU and 2x560's.
And a point of fact: although it's often the case, there are definitely other reasons for lower than 99% gpu use besides CPU bottlenecking.
OP your fps in one game to the next makes no difference. Dirt2 is extremely well optimized for the most part, and I wouldn't expect less from 3. Just because one games runs at something doesn't mean the next game will equal that. I'm willing to bet that someone with an iX would get better performance with the same card though.