I'll also agree with Arch13's TigerDirect recommendation.
If you get their catalog, it has handy charts that shows which RAM, CPU and mobos go together.
Also, a great resource for picking a motherboard is
www.motherboards.org. They have a ranking/review system you can browse by platform (AMD or Intel).
My personal recommendation is to get the Abit AN7...
It's got great onboard sound, and is super for gaming & what-not. I've been very happy with mine (from newegg). Get a beefy power supply, though. 400w may just barely cut it with a good video card.
Good luck! Don't be too intimidated - - building a PC is easier than you think, and you'll save a bundle!