Upgrading my computer

i currently have a msi gtx 660 ti power edition card in my system, but i want to know if it would be better to sli or go with a gtx 780?

gtx 780 would be faster but you have to do your math to see if its worth the extra cost over another 660

 

ok, now is it ok to put a 3gb 660ti with my 2gb 660ti?

its fine, but if i remember correctly it will only use 2 gbs of each card.