You need more v ram to run multiple screens. A single monitor can go over a gig on occasion. so if you have 3 monitors 3g of v ram might not be enough.
If you are playing on 1 monitor though Shadow is right. There is no reason for more v ram.
The msi one is out. why not just get this.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127797&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
why on earth would you buy a 6GB card over a 3GB card? You get almost no benefit, other than future proofing for a slightly longer time. 3GB is PLENTY enough for gaming at ultra with AA.
You need more v ram to run multiple screens. A single monitor can go over a gig on occasion. so if you have 3 monitors 3g of v ram might not be enough.
If you are playing on 1 monitor though Shadow is right. There is no reason for more v ram.