A quick correction to this article:
http://www.itreviews.co.uk/hardware/h971.htm

SLI can be enabled when two cards of the same type are paired and connected. The artificial disabling of the use of other manufacter's cards was removed from the NV drivers a while ago.

The machine I'm using now actually has a BFG 7800 GT OC and a Leadtek 7800 GT installed and is happily running SLI no problems, giving a fairly healthy frame rate boost whilst still allowing me to run at 1680x1050 AND enable all of the eye candy.

Regards,
undergroove