Basically, your looking for a 775 processor socket motherboard, with 1333mhz support (The Quad core is 1033mhz, but 1333mhz will give you better support for future CPU's when it comes to an upgrade)
Your looking for one with a PCI-E express bus, for graphics, and possibly SLI support (though not a neccessity), if you will ever want to run 2 of those bad boys in 1 rig... (money anyone?
Your looking for one with DDR2 support (There may be boards that support DDR2 and 3...it may be worth looking out for one, again, for future proofing)
Most boards also come with IDE (older but still used) and SATA/SATA 2 support, so hard drives/disk drives won't be an issue, and boards come with about 3 PCI slots, making network cards/sound cards compatable, etc.
Oh, and go for an ATX model to fit your case
What I would do is, find a couple you like the look of based on the specs above, and then look at a few reviews to see which performs better at tasks, and research that one more, and finally buy it.
You can pay tonnes for a motherboard, but cheap isn't always the worst option, but, to get the right one is very important.