i7 will be more for future proofing. Phenom II 955 should handle 5870 crossfire pretty well, especially if you OC it.
I'd get an i7 tbh, it would last you quite a bit longer, and I think is worth the price.
If you have an AM3 mobo already (says your config) that you would use then I'd just go the Phenom II route. That can run a 5970 well (when overclocked especially) and your mobo is stuck with a single PCI-E it looks like.