Since the computer isnt going to be hitting games too hard, and i imagine you arent too interested in extreme overclocking, you might want to look at an integrated solution.
There are lots of choices for Micro ATX Nforce2 IGP fully integrated (Video + Sound + LAN) boards that will actually fit into your price range. If Micro ATX wont cut it and you need all the PCI slots you can get,
Abit's NF7-M will probably fit the bill.
As far as which Nforce2 mATX board to get?
Shuttle's MN31N is at a medium price point, has the MCP-T southbridge with SoundStorm (the board isnt FULLY soundstorm compliant though, as there is no seperate subwoofer plug... but it IS SoundStorm quality audio). On the low end of the price spectrum,
Biostar's M7NCG 400 seems to be fairly solid, as well, and is under $70!