$350 is really not much to work with when your aiming for something with a upgrade future, if you do not have a copy of windows and a case with a some what current power supply your options are pretty limited.
1. Barebones from Newegg, Tigerdirect or other chain.... usually a semi crappy MB with not much future.
2. Check Craiglist,Ebay and local classified and sources like http://www.techpowerup.com/forums/forums/buy-sell-trade-giveaway-forum.43/, http://arstechnica.com/civis/viewforum.php?f=12, http://www.tomshardware.com/forum/8/classifieds.html, http://www.hardwarecanucks.com/forum/buy-sell-trade/, for used parts that you may pick up very cheap.
3. Save your money and aim for around $500 and build something with a little more potential, I'm not a big fan of "Budget" computers MB with no overclocking and sockets with not much future potential just don't seem worth the effort when your frustrated with crappy frame rates less than 2 years down the road.