Right now, 8600 at 3.33ghz will play most games faster than q8400 at 2.66ghz due to its faster clock speeds as most games are still only 1/2 core optimised, but a few games a re multicore optimised like supreme commander ect, where the extra two cores on the 8400 will put it in the lead.
Alos depends on your GPU setup, if you go CF 5770 or greater or sli 260 or greater then Q8400 is better.
The e8600 will overclock to 4ghz with good cooler and Mobo that can go fsb 1600 easily and the q8400 will go to 3.2ghz on 1600 fsb. (multiplier 8 * fsb 400 = 3200mhz)
If you have a good Mobo (p43 45 x48 chipset or good x38 p35) with good psu you are probably able to get that q8400 up to 3.8ghz ( 8 * 475 = 3800 mhz) but that is if you have good cooling and Mobo that can overclock with good Ram dividers in bios and good ram + stable power.
over all I would side with the q8400 as time goes on and more applications are coded for Mulit core it will last longer in my opinion.
Are you buying a new computer or upgrading an older one?
If you are buying a new computer then NEITHER would be a good buy, I advice you to consider Core i5 750 or Phenom II X4.
If you are upgrading an older LGA 775 motherboard then the Q8400 might be a decent purchase (depending on what are you upgrading from), but the E8600 is a poor choice with low bang to buck ratio and I don't recommend it at all.
i'm upgrading my previous g41 msi mobo..and x48,x58 are very expensive in my country. but some charts shows E8xxx (6 mb cash) are faster than Q8400 and also shows similer to q9650 (12 mb cash)
it feels like to me is core2duo better than core2quads.....
please explain this..how can i belive those charts