with 3 LCD at 27", your min res would be 5760x1080.
with 3 LCD at 30", your min res would be 7680x1600.

if you intend on running any 3d games, with any nice eyecandy settings, you need the 2 GB vram cards

now you are choosing between 5850/70 and 6950/70. can you get good performance out of a 5 series? yes. if price is comparable to 6 series, why bother? get 6 series. now, 6950 or 6970? if you don't want to oc, get 6970. if you can oc, get 6950, oc to 6970 performance levels, done.

at least, that's how i see it. good luck!