Depends on your budget - and how long you want to keep the machine the same without upgrading. I've not got 3 monitors myself so I can't recommend it but a lot of people with 3 are very happy.
If you're not short of a bob or two then just get the 4GB version IMO. Then you can put the VRAM question to bed (triple monitor wise) and when the 680 starts to show its age you can get another and go SLI if needs be
2GB is fine for now though if you don't mind changing it around at a later date if it needs to be.
Well i kind of keep upping my budget, which is a slipperly slope.
I was going for a 1.1-1.2k build with a 2500k / 570.
Decided to wait, get Ivybridge, it was at this point i decided i could use the extra cash on a 680, i don't particularly want to add an extra £100 on the card, if im then buying 3 monitors... and having to wait for the card ofc (i've been without a rig for a few years).
Having said all that i want the system to last a few years without NEEDING upgrading (if i choose to later thats a different matter)
Some stuff for me to think over... thanks for all the help.