Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
I would say yes, multi-GPU setups like the 5970 like to have a top CPU to push them so the more Ghz the better, the sooner you upgrade the better.
Yeah, by then ATI should have some new cards out and there is NV of course, it's a while to go to October so it's tough decision, of course you will see a massive performance increase going with a 5970, just you wont see its full potential until you upgrade or even OC your CPU.