I've decided to go with a DDR3 mobo with an Athlon II X3 425 (Rana).
I'd like a mobo that I can upgrade pretty well for the future. I'd also like to do a little OCing.
I have a slim (low-profile) HTPC case, with a Radeon HD 4350. I'd like to do some light gaming (mostly TF2, some other Source games, BioShock, Mass Effect, Prototype), be able to playback 1080p, and do multitasking office work (internet and Office 07)
From all the reviews I've read, the GIGABYTE GA-MA785GMT-UD2H is a great mATX mobo for OCing. However, Newegg has a deal which ends up being $20 cheaper with the ECS Black Series A785GM-M. From all the reviews I've read, the ECS board has ok OCing options, but has lots of driver conflicts and general errors.
I don't plan to OC much, as I have a small case and a stock cooler and don't plan on replacing it. Maybe just the RAM and a little bit of the processor.
Depends on your experience building systems. Believe it or not, ecs might be easier to deal with for warranty. Gigabyte is a royal pain. If you do a fresh install, you should have no problems with the ecs. Be sure to send in any rebates right away. I've gotten all of mine, but they take awhile to process. I don't overclock; prefer stability.