In that price range you'd probably have to build it yourself- Or have a computer shop do it, although their labor cost usually adds $100 to your final price. Also, don't forget that a copy of Windows 7 adds another $100, most DIY guides include the price of hardware only.
For pre-built systems, you basically have two options in your price range:
1) Buy a low-cost starting computer and upgrade the power supply/graphics card yourself.
2) Buy a CyberPower PC system. I think they're higher quality than Ibuypower, although the two companies seem to have merged or something.
If you're going the self-build route here are some nice guides:
http://www.tomshardware.com/reviews/build-a-pc-overclock-athlon-ii-x3,2811.html
http://www.hardware-revolution.com/budget-gaming-pc-q4-2010/
If you're looking for something off-the-shelf to upgrade:
http://www.walmart.com/ip/GUA103SE/15231528
Then add these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139018
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161338
And you'd have a nice gaming PC, under budget.
This is an even better starting PC:
http://www.walmart.com/ip/CyberPower-Cyberpower-GXI107/14860537
But you wouldn't be able to upgrade it as much without overshooting $700. The best video card you could install with the stock power supply is a Radeon 5670:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102871
But I'd recommend the same upgrades I posted earlier. If that's too high, go with these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139017
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125342
Keep in mind that 75% of your gaming performance depends upon the graphics card, so having the first system (Athlon II) with a Radeon 5770 beats having the 2nd system (Core i3) with a Radeon 5670 or even 5750/GTS 450. In very CPU-intensive titles like Far Cry 2, the Core i3 would give you a boost, but not enough to offset having a weaker graphics card.
http://www.techspot.com/review/240-ati-radeon-hd-5670/page4.html