Your motherboard supports quad core CPU's alright.http://www.asus.com/Motherboards/Intel_ ... 43TD/#CPUS
Your'e right about people who sell these CPU's though, prices are a bit high for old 775 quads.
If I where you I would get this combo instead:
ASUS P8H61-M PRO mATX (If you want 16GB of RAM getting 4x4GB can be 10-15USD cheaper than 2x8GB at 1333Mhz, hence full ATX would be necessary)
Intel i5-2310 or if you wanna overclock then the one you chose is nice.
One can also make a pretty cool and very space efficient rig with mini-ITX motherboards. Did one recently for a friend of mine.
i3-2100 | 6950 2GB | 64GB SSD 2.5" | 1TB HDD 3.5" | Blu-ray slim drive | 8GB RAMhttp://www.xtremesystems.org/forums/att ... 1332330750
Ivy-Bridge is getting closer to launch, this might change the pricing a bit.