I can get you a halfway decent laptop for gaming for $700. $600? Probably not.
For about $650 you can get an Acer Aspire 7741G-6426. It's got in intel i5 processor, a radeon HD 6550M graphics card, an 17.3 inch display, and a 640 gig HDD.
It's decent at gaming, will run Metro 2033 and Crysis on high settings at a playable frame rate, and will run things like Left4Dead or TF2 with absolutely no problems whatsoever.
The problem is that the computer runs hot, so you'll need a cooling pad, the HDD runs at 5400 rpm, so that'll be a little bit slow, the battery is only a 6 cell, so it only lasts about 3 hours when running on minimum power, and only like 45 minutes when actually gaming, and the parts aren't super high quality, so don't expect the computer to last longer than 2 or 3 years.
You really have to make quite a few compromises and sacrifices if you want decent power at that price, but you can definitely have a laptop that's decent for gaming.