Most folks here will recommend you assemble your own gaming PC. Once you have the components in mind, it's basically just putting the parts together. You'll need:
Operating System (Win7)
If building your own is out, I really have no experience with either pre-built brand. But lots of folks like the ibuypower. The last pre-built I bought was a Velocity Micro. Those I can vouch for as being a well made PC built in the U.S.: http://www.velocitymicro.com/category.php?cid=12
If you are interested in a slightly used gaming PC at ~$700, PM me and I'll link you to one I have on ebay. I built it earlier this summer.