Ok, go with a ~2.6-3.0 Ghz Intel Core 2 Dual Core CPU. For budget gaming, a quad simply isn't a good option at this point. 2.66Ghz can be had for $130.
Video cards... ATI has a slight lead now in higher end performace/$, but its more or less even in the budget area. Go with a 9600GT or if you really want an ATI, a 3870 or a 4670 (haven't heard much about this one though). About $80.
2Gb of DDR2-800 ram... about $50. (G.SKILL has worked for me well)
Socket 775 motherboard... ~$130-140. I agree that a 680i is a good choice (Make sure you get a good quality one)
Case: any good midtower ATX will do ~$75
PSU, get a good (Corsair, PC Power and Cooling, OCZ) PSU of about 500-600W. ~$70.
Hard drive: this one has served me well, 640Gb, $75,
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218
DVD drive $30.
Windows (*#$%$&@) ~$100
All that comes out to $745, so with any new monitor, speakers, keyboard etc (and anything I forgot to put on the list) you should come in around budget.
This all assumes you're into building computers. (Its the way to go man, you can do eet!)
This should run Orange Box at near max. And if you want to do better than that, you'll need more moneys.