If the i7s come into play, then the i7 860 is probably the best option for rendering. If you're using huge files, then the i7 920 has a slight benefit from the triple channel memory. The i7s should still beat the unreleased x6s.
Not sure if you're using a workstation card or not, but this is worth a read (intro and conclusion for sure) if you are using a gaming card:http://www.xbitlabs.com/articles/video/display/quadrofx-firepro.html
If you can't afford even the entry level nvidia cards, look into soft moding the bios of a gaming card.