[Presto]

My sister is looking for a machine capable of running games like Left 4 Dead. I can probably slap something together, but some recommendations would be appreciated. The budget is $500 before taxes. Don't need screen or hard drive. Thanks, folks.

[Eclypz]

Here are the requirements....the Source engine doesn't need anything powerful.
http://www.left4dead411.com/left-4-dead-system-requirements

[Presto]

I'm aware of the requirements. I'd go for the cheapest parts to get the job done. I would like opinions on the best value under $500. I'd probably go for the cheapest Asus board (~$80), and an E5200 ($90). I'm not sure what there is for video. I'd go with either nvidia or ati.

[asian_XL]

just buy her an used P4 for under $200