If you live in the US in a city with a Microcenter you can get some DAMN good deals for putting together your own system. Their CPU prices are at or lower than newegg and they always have combos with workable motherboards for as little as $10 more (and up, obviously). Keep cruising newegg/amazon for deals on a decent power supply, hard drive and memory, the cheapest case you can stand (check reviews to make sure it will fit a long graphics card), and then use all the money left for a graphics card. Obviously if you need a monitor or Windows factor that in.
You can put together a pretty damn beefy gaming system for $800 especially if you already have the monitor. Example: i5-3570k $169.99 + Asus P8Z77-V LX $69.99 at Microcenter, $60 hard drive, 8G memory for $25-30 or so, $30-40 for whatever decent case newegg has on sale, whatever decent PS on sale - don't try to go too cheap on a PS, look at jonnyguru.com reviews. I usually try to get a Seasonic, but newegg has an Antec Earthwatts EA-650 for $30 after rebate
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371044 which should be decent. Or a Seasonic G 550 for $55 after rebate
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151119. So far $400 w/Earthwatts, leaving roughly $400 for Windows and a graphics card. Even 7970's have been on sale for $300 after rebate recently, you could definitely step down from that and still have a pretty monster system. Here's $330 after rebate, with a pretty decent bundle of games:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202008