Why not some 200 to 250 buck laptop? Here's what folk are buying.
http://www.amazon.com/Best-Sellers-Computers-Accessories-Laptop/zgbs/pc/565108
http://www.amazon.com/Best-Sellers-Electronics-Desktop-Computers/zgbs/electronics/565098 if desktops are your thing.
Gaming? Desktops? on the cheap? AMD A10 desktops such as http://computers.woot.com/plus/get-your-asus-to-mars?ref=cnt_wp_9
Bob
I've using a 6 year old budget PC that I've upgraded (memory, graphics card, RAM) through the years...but it's getting to the point where I have to buy new. The main use would be as a Plex server. What would be my cheapest, but best bang for the buck, option? I wouldn't even be opposed to one of those small box PCs. Thanks for any advice.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic