I think your best bet is to build it yourself, you should be able to get a low end dual core with 2gb of ram and cheapo video card for under $300.

Could always buy a barebone PC (case, motherboard and powersupply) for ~$100 pop in a cheap dual core for ~$50 and some ram fror ~50 and a cheap video card (or just use on board video if mobo has it), cheap CDDVD and be out for around $300.

If you want recommendations on parts
http://www.hardforum.com/index.php is where I get all my computer needs, if you feel comfortable building a comp yourself (it really is not hard) I would ask in the General Hardware sub forum there and they can recommend a good setup and you can order everything off newegg or what ever site.