Think you'll need to run then at same core / mem speeds in case they're not. I've never had SLi and I don't plan to so I don't speak from experience, but it would make sense.
mmm... I'm just wondering because i bought one card hoping to upgrade to two but then they dissapeared. Now dx10 is on the horizon and my single gt seems to work fine for now, but sli g80's really need a conroe so the only sli future would be with the g71s. oh well. we will see how things go.
If you bought your board within the last year you should be fine as far as different brands as long as they're the same card running at identical speeds. Unless you need the performance boost now I'd just wait and see how DX10 cards shape up.