i would get the E8400 over the Q8400. the Q8400 does not OC as well as the E8400 and most games still rely on just two cores and will until the next generation of consoles come out. By that time, both CPUs will be outdated for gaming.
If you can get an E8400 for $90 then go for it. With decent cooling it will easily go to 3.6 on stock voltage and over 4ghz with a slight bump. I am still quite happy with mine even on games that can use more than 2 cores like Dragon Age I get 75FPS+ with my E8400/GTX460 1 GB combo. A Q8400 will not overclock anywhere close to that.
I agree I had an E8400 and it was an overclocking monster, with water I got to 4.6ghz prime stable with a single GTX 275 and had a 3dmark06 total score of 19676 and a CPU score of 4276. E8400 all the way.