x8XX > x6xx
The 9600 would be best for multiple rigs just because of the price point and its a good Mid-Range.

The 512 version of the 8800gt Out performs the 1gb version in most cases.

http://en.expreview.com/2007/12/30/r...need%EF%BC%9F/
and
http://www.firingsquad.com/hardware/...iew/page16.asp


But i'm not sure how the 1gb version would compare when running 2+ copies of wow on one machine.