What nobody here has said is that theres a possible compatibility issue with that mobo and 45nm quads, and to a lesser extent the duals as well. The latest 'official' bios for the p5b, version 1236 is a bit iffy, but the 'beta' 1237 seems to do the trick. Check page 196 of this mega thread on the p5b at xtremesystems.org:http://www.xtremesystems.org/forums/showthread.php?t=112648&page=196
Your in a similar position to me, ive got an e6300 and a p5b deluxe. I'd like a q9650, or a q9450 or e8600, but the costs are soo high in the uk right now. Id like to stick a really really good cpu in this old board, but they cost a packet. The q6600 is a pretty good substitute, but it sucks down power. Hopefully 45nm quads and duals with the full 12mb and 6mb L2 caches will be alot cheaper by the summer (when i plan to make a decision and upgrade my cpu, get more ram and get vista 64) cheers.
In short get the q9450 and google 'asus p5b bios 1237'.
or try this link for the download lol:http://vip.asus.com/forum/view.aspx?id=20061223034227274&board_id=1&model=P5B+Deluxe%2FWiFi-AP&page=1&SLanguage=en-us
If you get the q9450 pm me and tell me how it went, cheers