790i is running my Q9450 at 3.4 on stock auto voltages (did bump the NB up a fraction and unlink the ram) right now.
Did 3.6 stable, but temps started creeping up after 30 minutes of prime 95, so backed her down a tad.
DDR3 is faster, but not a noticeable difference in most apps, BadDad linked a really good article a month or so ago extolling all the virtues of the DDR3 vs the 2, one reason (and the new NB) that I moved to the 790i from my 680i. Hard to justify the same 4 gigs I got would buy 16-24 gigs of ddr2.
Just be sure to update to the P5 bios before overclocking, and as I said- unlink the fsb/ram, bump NB to 1.4, adequately cool the cpu and BAM! It does support the Q right out of the box, but my board came with the P3 installed which was less stable than the P5. Almost a month and not one BSOD yet.
Only thing wrong with the board is the NB fan sounds like a chainsaw, cut it to 40% in the bios, and blessed silence returns.
I am on water for the cpu and my 2 vid cards sli'd, so the NB was the only noticeable noise coming from it.