i've found that not all SLI config's react the same.

Back in the day, i had a pair of 6600gt's in SLI. Diff makes, one was clocked slightly higher than the other. After much trial and error, i could only get a stable bridge when both cores were clocked the same. That's why i threw that out there.

Bork's right tho. Instead of another 7950, save a little and go with an 8800GT or Gts. it will out perform two 7950's any day.