HD 7990 - Ultra fast card but you wont have physX
Gtx 680 - Like mr.s and darkol said its a headache..(Heat Power) etc
Gtx 690 - Its basically like slightly down clocked version of 2 680's..but as far as benchmarks goes it beats the titan..
I would recommend you get the 690, I had a multi gpu card before 980gx2.. it was an amazing card at the time, I was able to set all games in ultra graphics without any problem at all..
hd 7990 > Gtx 680 sli > gtx 690 > titan
HD 7990 - Ultra fast card but you wont have physX
Gtx 680 - Like mr.s and darkol said its a headache..(Heat Power) etc
Gtx 690 - Its basically like slightly down clocked version of 2 680's..but as far as benchmarks goes it beats the titan..
I would recommend you get the 690, I had a multi gpu card before 980gx2.. it was an amazing card at the time, I was able to set all games in ultra graphics without any problem at all..