if you can afford 760 4GB SLi, then go for 770 2GB Sli, almost the same price with better performance. unless you are have/planning on more than 1080p or multi-display.
Is it really worth it, can 760 use all 4 gb of VRAM or there will be no difference between 2gb & 4gb sli.
4GB
You wont get better performance but next gen here and what if you want to upgrade to a 1440p monitor or if games go over 2GB such as skyrim mods/demanding games you will be in a better position. Btw 760s in sli is pretty slick i havent seen any micro stuttering/issues since ive ran them and well worth the money.