Ditch that PSU, its garbage, look at antec, ocz, corsair, silverstone, seasonic, PC P&C for quality PSU's. For gaming, an E8400 or PII 720 would be better options than the Q8200, the latter being a better option if you like GTAIV or FSX. Seagates are also plagued with issues, stay away from them. Western Digital is leading the industry in HDD quality, second is probably Samsung.
Read this: http://techreport.com/articles.x/16472/12
Seagates do tend to have faster read/write speeds, but the other issues that go along with them are not worth it IMO. They still need to work out their firmware. Samsung drives on the other hand have fewer problems and are quite fast as well.
Honestly, as for the hd's the only problems I've seen lately has been my brothers 250GB WD, certain programs would not run right off of it/cause system crashes all the time. He checked it part by part, and the issues were only resolved with a different HD. As for Seagates, I have a 500GB main drive and a 160GB secondary for storage. Neither have had any flukes whatsoever. My brother also replaced his 250GB WD with a 500GB Seagate. The 160GB seagate has run without issue for almost 2 years, and the 500GB for several months since I got it. I wouldn't stay away from Seagate if there is better perfromance or price.
Yes, it runs cooler and more efficiently. If you can shell out a few more bucks then go for it. The Q6600 can OC a bit higher, but you need watercooling to really take advantage of that because it runs much hotter.
Depends on the games. Most games will not use all 4 cores of a quad so Dual's are usually better. Some games like FSX or GTAIV and some RTS games will use quad core CPU's, in which case the Q9400+GTS 250 is better, all other games would point to the GTX 260.