I would suggest the 45nm quad cores if you are gonna do the quad core thing...same with the dual cores. Quad would be Q9450, or Q9300 for cheaper variant...only slightly more than a Q6600. Personally, I would jsut go with an E8400 dual core 45nm at 3ghz...perfect...plus they are cheaper than the ones you linked above, and are the same speed/faster.
Also, would suggest the intel x38 chipset. Just watch which one so you get DDR2 usage. The gigabyte EX38-DS4 version is very good and on the less expensive side. Of course, that would NOT allow you to do SLI. If you are deadset on SLI, you have to go with 750i/780i chipset...which is a real stinker with OC'ing and whatnot...and HEAT. Since you bought DDR2 memory, 790i chipset is OUT for you.
Just remember, the X38 is equiped for future 1600FSB processors from Intel, while the 750/780i chipsets do NOT officially support it (You would most likely just have to overclock the board).
750i northbridge FTW 750i is getting nice reviews.
780i northbridge Not considered "great" by the OC community..but whatever, that may not be your thing.
Gigabyte EX38-DS4 Nice reviews, but Crossfire instead of SLI.
I wouldn't bother with an expensive graphics card right now. The GX2 is already gonna be on its EOL in a month...very short cycle, and replaced in June I think...but its all rumors now on when replacements come out, really. Considering the prices on 9800GTX cards have just dropped below 300 (evga included...step up program), they are very inexpensive for the money...and that card OC's like a monster. The 8800GTS is also getting damn close to 200 bucks before rebates which would be an outstanding deal.
Personally, if you are going to use it for gaming, I would build an AMD system to save on the cash

Nvidia is so right with their statements of GPU make gaming, not CPU...Just buy the best amd motherboard you can afford, and just an ok CPU, and wait for some good phenoms to come out. The 3 core phenoms come out in a week or two.
My only other opinion to share, would be, if you can wait another year, do it. Intel will be changing sockets with its next chip, which will come out late this year/early next year. new everything...with internal memory controller like AMD does. edit: Woops, you bought the ram, so build the system
