Does anyone have a trick to get the 2nd power supply turned  on  as to power the 3rd and/or 4th card?

I have a 1200 watt power supply, but want to put 3 5850's on one mother board.

The only thing I can think to do is grab an old mother board, but then the old board might not be compatable with the 20 pin connector. ( my old boards are....well old.)