If you are not planning on waiting until Q1 2011 (which is a while), the i7 950 is a great setup < $300. Although, better value can be found in i7 860 $199 or i7 870 for $229 at microcenter. The performance difference between PCIe 16x/16x and 8x/8x is immaterial.
I think 5970 is a horrible value. GTX460s overclocked/470s or 5850s in CF are better value imo. If you are looking at the 5970, then surely wait for HD6000 series in November. You will also need to upgrade your PSU for dual card setups (aside from 460s I imagine).