If you don't mind used parts and having some patience you can get some amazing deals on Ebay.
http://cgi.ebay.com/Ultimate-Gaming-...#ht_500wt_1154
For instance.

Now what I ended up doing was buying my case that already had 3.4ghz dual core processor, a 500gHD, dvd burn drive, and 4 ram slots. Then I bought a used Nvidia card. This thing is a beast! It's probably a cycle or two behind the new cards but I read old reviews on it first and got it on a great deal. I happened to luck into getting Vista right before Win7 came out, so I got Vista on the cheap, and they included a free upgrade for Win7. Found a good deal on a nice 23' flat screen and I'm set. I don't have all my specs now but I put it together a year ago. Set a nice gaming on the cheap for 500$.

Just gotta get on there every day and keep your eyes open. I got the operating system and more Ram off newegg.