Being on a budget, buying a 780i board & wanting to do Tri-SLI really don't even belong in the same conversation together...
First, I would determine what a realistic preliminary budget is (ie, $1500 - $2000)...After you come up with that number (as magical as it may be), I would carefully select the board...I've read a lot of great reviews about the EVGA 780i board & the Striker II (I'm doing my install this week)...(Striker II is a very expensive board...If I had a lower budget, I would easily have gone with the EVGA board)...
Next, you'd want to determine which graphics cards you want to end up with in SLI...Right now, you can only choose between the 8800GTX & 8800 Ultra...If the 9800GTX gets released, that'll be an option too...
Make sure you plan well ahead for all of your components, not just those mentioned above...You need a damn powerful power supply & a large enough case to house everything, along with adequate cooling...Start there & see how far you get...
To be continued...