Not all 775 socket mobos will support the new quad cores unfortunately.
I'm an SLI person (2 Nvidia graphics cards) so I have to get a 780i because the 680i doesn't currently support the new quads.
I'm fairly sure all the newer intel boards take the new quads. If you don't want SLI, a P35 board would be fine (as jermaghs07 suggests, though that link takes me to a RAM page...). From what I hear the X38 is not much better than the P35. The X48 is an unknown at the moment (to me anyway)
If the link doesn't work it is an ASUS P5K-E LGA 775 P35 ATX.
This is the same board i have in my computer, but i don't use SLI. i just have one single BFG 8800 GTS.
Personaly i think quad core is a waist of money. you can get alot faster dual core for half the money. unless you plan on running 10,000 programs at once you don't really need a quad core. I would stick with a 3Ghz Dual core.
Quad cores were developed for servers, and to put one in a desktop is not practical, it's just showing off.
Currently quads are not utilised (unless you're doing heaps of things at once or playing Crysis) but it's about future-proofing. I don't upgrade very often, so when I do, I'll be getting a quad (Q9450).
The new Q9450 is essentially a replacement for the Q6600. It's not going to be a thousand dollars, more like 4 or 5 hundred. The extreme versions are for people with too much money.
I think you're spot on with the P35 board, though a sensible person would google it and read a few reviews before buying.