Agree with Major_A, if the Q9400 is cheap and you are only looking for a minor speed boost, as long as it is overclockable. I don't think you can find them retail anymore as they are discontinued.
That said for $350 - $400 you could build a barebones Core i5 2500k system with 4GB RAM that should be very easily overclockable.
Of course none of my drivel answered your initial question, I think that with a BIOS update that it should work fine.
Asrock Z68 Extreme4 Gen3
Core i7 2600k
Corsair H100 Liquid Cooler
8gig (2x4) G.Skill Ripjaws DDR3-1600
Powercolor HD5870 PCS+
OCZ Agility 2 240G SSD (boot, programs, games)
WD Caviar Black 1TB (Data)
CM UCP RS700-AAAAA3 700W 80+ Silver PSU
CM Storm Sniper
Win 7 Pro 64bit