I would go with the 670 - there is even a 4GB version of them so you could SLI and still have access to 4 gig of video ram. That's what I call destruction
4GB not really necessary even at high resolutions.
the 4GB -- Realistically there was not one game that we tested that could benefit from the two extra GB's of graphics memory. Even at 2560x1600 (which is a massive 4 Mpixels resolution) there was just no measurable difference.
Now the setup could benefit from triple monitor setups at 5760x1080 (which is a 6 Mpixels resolution), but even there I doubt if 4 GB is really something you'd need to spend money on. It might make a difference at 16xAA and the most stringent games, or if you game in 3D Stereo and triple monitor gaming -- I mean sure -- at any point graphics memory can and will run out. There's one exception to the rule, and that's Skyrim all beefed, tweaked and modded upwards. But the universal question remains, is it worth it investing in that extra memor? This card is 90 EUR more expensive. Well that answer depends on pricing versus your demands and requriements really, the extra memory certainly won't hurt that's for sure, but sure -- the benefits remains small.