The Q9650 is 2 E8400s together. The Q8400 is more like 2 E5200s (due to it only have 4MB of L2 cache) but with a higher FSB (which makes little to no difference). The E8400 in games would rock the Q8400 (at least in 95% of games), especially when OCed. Although, if you are getting an E8400 I really urge you to look into OCing. That chip is made to OC and it does it like a dream. May as well do an OC without raising voltages, the E8400 averages at 3.6ghz w/o raising the volts.
If you use quad core optimized programs then go for the Q8400, if you are gaming I'd go for the E8400 and a little OC.
About the power supply, you'll be wanting to buy your own. I'm guessing the one that comes in the deal is a generic power supply. The PSU is one thing you do not want to skimp on.
You sir, are correct. I was two quick with my answer and goofed up.