SLI setting causes failure

[perplexed]

Machineellxps720,Nvidia chipset, q6600quad, 4x1gbMem chips,1kw pws, Win Vista 32bit sp2.
2 nvidia cards one from Dell their GeForce 8800GT, and a PNY 8800GT, connected with Dell's-SLI bridge,

when boot up, machine works, with 2 cards. But, when activate SLI through Nvidia control panel, Both cards are disabled in OS, forcing a reboot with only 1 card to start over again.

 

[perplexed]

NVIDIA nForce 680i SLI if needed