You need a socket 775 motherboard.
What will you use the PC for? You can get a micro-ATX board if you aren't looking for tons of power. I have this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131680&cm_re=asus_775_motherboard-_-13-131-680-_-Product
It's a great board with decent on-board video. You won't be able to play games at high details or anything, but it's a great board for a productivity machine. I run development tools and photoshop on this machine with no problem.
It's a good solid basic board with decent on-board video. I've since added a Radeon 5450 which boosted my 3D performance, though it actually took away from the 2D rating in the Windows Experience Index.