I went back to BIOS 0803 after reading alot of Q9550 users found the best stability overclocking with it. I used the Kodakey bootable CD and had no issues flashing backwards, Alt-F2 wouldn't let me obviously.

I've gotten stability at 4Ghz after alot of messing about, I was ready to throw in the towel last night a few times and just stick the P5K-E back in. Then I found a post where someone had suggested some really high voltages for NB, SB,etc. Starting from there things were stable, and I have worked my way back down maintaining Prime95 stability now though. I think the biggest factors were the CPU GTLs to 0.68 and NB to 1.3v (backed off to 1.28v now and still stable, previously I don't think I was giving it more than 1.24v though). Maybe more of it is the BIOS too, not sure, either way I can blend and small FFTs now.

GTL .68/.68
PLL 1.52v
FSB 1.22v
NB 1.28v
NB GTL .67
SB 1.20v
SATA 1.50v

This ran prime 95 last night for about 2 hours. Just starting it up again today. Let me know if you guys have any thoughts on the current voltages I'm using.