1GB and 2GB graphics card. Is there any difference?

[carnage]

To avoid wasting your time, here's the relevant (performance-wise) part of my rig:

CPU: Intel Core i5-2500K @3.3GHz
RAM: Corsair Vengeance 1x4GB DDR3 1600MHz CL9 (so yeah, Single Channel for now, but I will upgrade with another 4gb module next month)
Hard Drive: WD Caviar Black 750GB 7200rpm 32MB cache
Mobo: ASRock P67 Pro3 SE, LGA 1155 socket (allows quad-crossfire, but not SLI, so I won't consider nvidia cards)
Power Supply: OCZ-Z1000M 1000watt 80+ gold modular PSU (so power-hungry cards aren't really much of a problem)

Oh and of course, I'm still building it. These are just the pieces I ordered. I'm on a tight budget, but I got the PSU for a "bargain" price (~100 euros) lol.
I won't be playing at resolutions higher than 1920x1080 and I won't be playing on multiple monitors either.

Now onto the real question: let's say I have do decide between a 1GB HD6950 and a 2GB one. Will I actually notice any difference in performance in newer games (i.e. BF3, Skyrim, MW3 and so on) at high or ultra(!) quality (what I want is stable >60 fps anyway)? If not, will the 1GB card still be enough after a year or more? I don't want to find myself with an obsolete card after only 6 months/1 year, even though I'm sure it'd be a vast improvement over my old entry-level 512MB DDR2 card. I plan on running a crossfire setup in the future, so yeah I want something that lasts at least a couple of years, but the less money I spend the better. I want to know if the extra GB is really worth the extra price (30-40 euros). If so, I guess I'll have to wait a bit until I get enough money for it and use my current crappy card for another month or two (ugh!). Or are there any other worthy (but less pricey) alternatives that I should consider?