2 or 3GB Vram, 3 monitors + questions

okay so im getting either asus 770 2GB or asus 780 3GB and im running 3 1080p monitors.

so im wondering may i run into any problems with the 2GB one or does it not matter too much?

i think i prefer the 770 seeing as its almost half the price but i dont know yet.

i have been running a sapphire 7970 3GB so far but there was something wrong so i sent it for rma and got money back

 

Question 2: the Asus Mars 760 having 2 gpu's in it means that only one will be used when playing games not in full screen mode right? 

 

 

 

Ty for all replys

the 3GB 780 SLI is better but expensive. a SLI of  2 770s is more better in terms of price assuming you are gaming on 3 monitors. 

The Asus Mars 760 is running as in sli but you can turn one gpu off (but i doubt anyone would want to)

im not getting sli cards, its a single 780 or a single 770, or possibly the mars card but likely not.