I've just built almost exactly the same spec as your original post and it's a fantastic machine. I went quad because I use it for audio recording and my program is optimised for quad cores. I'd still say go quad if you do anything remotely CPU intensive because it means you have more options to run multiple processes at once.
The ASUS board is okay, but I think I'd go ABIT if I were building again because the board layout of the P5K premiun isn't that great. The SATA connectors are totally in the wrong place if you've got a longer Gfx card like a GTS. I had to remove the cover on my card and it was still a squeeze, definitely better off going for a board which had the connectors further down. If you don't remove the cooler casing on a double slot card like a GTS, you probably won't be able to use 2 (possibly 4) of the 6 sata ports on the board, even with right angle connectors (believe me I tried).
Couple that with the fact that the PCI-E slot is next to a PCI slot, you lose the PCI slot with a double slot cards. It's a fantastic board feature wise, but I wouldn't buy another if I had the choice.