Hardware for your budget:
Middle class mobo, single socket. Personally, i'd go with DDR2 RAM for upgradability's sake.
AMD 3500. Somewhere around there. Nice and cheap, decent speed. 87 bucks.
A big HDD. Seagate Barracuda 300GB minimum, i'd say. Plus at 95 bucks for SATA 2.0, it's an H-E-double-hockey-sticks of a price.
$30 DVD-RW drive would be more than perfect. You could probably get by fine with just a CD-RW, but with the price range so small, you'd be saving maybe 3 bucks. Not worth it, IMO.
Middle class video card. Nothing too expensive, but nothing too cheap.
I'd say forget a sound card. You more than likely won't use it.