I say the Intel P45 Chipset, hands down.
obviously what mobo you get depends on what else you want to do with it.
crossfire/sli/single gpu?
ddr2/3?
etc...
+1 The P45 based MBs offer high performance and excellent value. Asus, Gigabyte, Biostar, DFI all OC very well. Each of these companies will offer multiple models within a chipset family, with a variety of speeds and feeds. I have a Q9450 and I went with Asus P5Q Pro on this build. What I was looking for is a great OCer, CF support, FireWire, RAID, and a solid brand. A plus is that the board actually looks really nice. (I have a window on my case) The MB is black, has a heat pipe, copper heat sinks, nice layout etc. You want to see an ugly MB, check out Foxconn. Its a shame too because Foxconn makes really good products. If they made their MB look like XFX or eVGA MBs I'll bet they would sell a whole lot more of them!
Once you select a MB, I suggest you grab the MB's qualified vendor list (QVL) for the memory. This list will indicate all the memory, by speed, which has been tested and validated for the MB. Then pick your memory (the exact model number) from that list. That can save you so much time and effort.
