From a marketing aspect, I suppose the Q9450, or perhaps the Q6600. I'm guessing noobs (no offense to them) would be more attracted to a Quad over the E8400, although the dual core is arguably the better chip for gaming and definitely is a better overclocker.
The Q9300 is weak. It sports less cache than the Q6600, a 7.5 multiplier (although I'm not sure if those buying your rigs care to overclock), and only offers a 100MHz speed increase over the Q6600.
If these sell, build a few with the Q9450 and the E8400.
