the difference between the ds3 revisions is negligible from what i recall but the 3.3 seemed slightly better for whatever reason (google it if you really need to know). the 620w psu is slightly overkill especially since you're not going to OC at all, and psu energy efficiency drops when utilization is below half IIRC, not that big a deal but something to consider still. your RAM is just fine, but you should double check to make sure your mobo supports it, the ds3 has been very picky with certain RAM brands and certain voltages too, resulting in RAM not being compatible or not working because the RAM required higher volts than the mobo was giving by default, meaning some people had to use cheaper low voltage RAM to boot up the system and manually raise the voltage then swap to the newer RAM.
and as for the 640 vs 320mb, 640 may be a bit overkill for the games played. the higher RAM proves useful for supporting higher resolutions and more intensive games but you are using a widescreen so it might not hurt... the only thing i'd recommend is a newer p35 motherboard because of the somewhat better chipset.
EDIT: mrfatboy's correct, 533ddr2 would do you just fine too, but since you're not OC'ing i think using your mobos highest rated RAM would be a good idea, and if you were wondering micron d9 is popular because of OC'ing.
i also can't comment on the q6600 running much hotter but an aftermarket cooler probably wouldn't hurt to keep it cooler, you're plenty below budget too. be careful with tower coolers though, they're huge and heavy, and don't fit in all cases. if you get one though, a one with a smaller footprint would be better. i got a arctic freezer pro 7 myself, it seemed to cool better on average than a few others and was just below that of tower coolers. anandtech did a review a bit ago which i used as reference too, just be careful when choosing a cooler since the direction it blows air matters, in my particular case i lucked out since my arctic freezer blows air directly towards the back fan of my case, but in another case it wouldn't have worked out so well.