Are you talking about the 6950? If it is, I would get the 2GB (reference) so you can unlock shaders. If not, and the card is purely for mining (e.g. no gaming) I would save the cash and get the 1GB. More details will help.
I suppose something like unlocking shaders might be worth the extra $10-15 it costs to go with a 2 GB card, if it would help resale value (in case this whole mining thing doesn't work out). But when used purely for mining I guess the answer is that it makes no difference, eh?
I was thinking either 6990's (best performance) or 6950's (much better performance per dollar). Thoughts on this?
Well unlocking shaders gives you more cores (1536 vs 1408) more cores = more hashes
I have a MSI 6950 unlocked shader and overclocked to 960 and get's about 410 MH/s. Now only if I could get it to run with my 5970 instead of sitting on my desk...
Be sure to get a reference card if you want to unlock.