Build your own..
You can build a hell of a computer for $1000.
Just don't make the dumb purchases that a lot of 'tweakers' like to do. More ram with slower timings is usually faster than less ram with faster timings. Faster timings are a rip-off, imo.
Also, same goes with hard drive. You're not gonna get a slow framerate if the hard drive is slow - you'll just have longer loading times. Why buy a 74GB 10,000rpm hard drive for $200 when you can get a 200GB 7200rpm hard drive for $140?
Same goes with CPU.. Buy a mid-range CPU. They're usually 1/3 as much as the high-end cpus. And, they're usually 9/10 as fast.
Graphics card.. buy a $200 graphics card. Don't blow $400 on a card that's only 10% faster than a $200 card.