For the proc the Q6600 is more future proof and offers better performance clock for clock, especially in Crysis or if you run background tasks. Unless you have really good cooling the E8400 will most likely out clock the Q6600 making it better for gaming as of now. Most likely the Q6600 will regain its crown over dual cores in future games. If you intend for this to be a long term build, go for the quad. If don't plan on having this comp very long (maybe 1 year) than the dual core will probably be better.
The ram sort of depends on how much you plan on OCing. If you don't plan on going past 3.6ghz the cheapest DDR2 800 ram will do. Otherwise, go for this ram.
The WD 640gb is faster and cheaper than the Seagate so no contest there.
Vista Ultimate 64 bit is the best gaming OS. If you want to step down to lower version you can but I bought Ultimate and have never regretted it neither has anyone else I know. The only people I know that dislike Vista are those on 32 bit versions.
Just get the cheapest sata optical drive you can find.
Case suggestion really depends on price. If you are willing to pay $300 I can tell you a good case. If you are willing to pay $80 I can tell you another. It is important to know the price bracket for this pick.
Your psu is a bit overkill but the psu is an are where you want overkill. If you have to save money this is an are where you can do it but the price for this psu is a very good deal atm.
The rest of your build looks great. So far you have picked good choices.