I would say go for an Opteron 146 because they OC like crazy or the 3700 sandy is a good choice too. I also would recommend the Lanparty DFI nF4 Ultra-D since it is what I use and I love it. It has a ton of options that are really nice to have. It isn't all that hard setting it up, a lot of people scare you but really it isn't that bad and once you do learn a bit more it is nice to have hardware that can allow you to do things. That added to the fact that DFI-Street.com forum has TONS of info on exactly how to do anything you would want basically. I would say go for that MOBO cause it is worth a little extra cash.
When you OC by increasing your HTT it increases the speed for the entire system including the ram unless you set a memory divider. The stickies have great info on how to OC and what effects what.
I recommend the Opteron 146 because I hear lots of people getting 2.9 ghz with it while I got stuck at 2.7ghz with my sandy. However it is true that people get over 2.9 with 3700 sandy's too, it is just luck of the draw. I got scrooed
Good luck, enjoy building, enjoy ocing, enjoy your new rig in gaming
P.S. if you get the 3700 sandy and the DFI Ultra-D PM me if you have any questions cause I have those exact same 2 things and have some good experience with em now
EDIT: If you are much of a gamer at all I would recommend getting 2 gigs of some decent ram. I can't tell you how glad I am I upgraded to 2 gigs of decent stuff when I am playing BF2 compared to my 1 gig of ValueSelect stuff that I used to have.