Here's my personal opinion on the matter and why I chose to buy a 4gig VRAM 970 without waiting.
The 8 gigs of VRAM will only be useful in the event that a game needs more than the 4. For most games that use under 4, you won't see any performance improvements - it will be a waste of money. For the games that use over 4, depending on the nature of it, you will start to see performance improvements but how much is still in question (there's a good Tom's hardware article about the 8gig 290 cards). The question will be, when would a game need more? Higher than 1080p resolutions, Ultra texture packs, etc. But if you start using these higher resolutions and ultra texture packs, you'll see a performance hit. I'd personally much rather have a solid 60+fps at 1080p than worry about my ability to his 4k resolutions down the line. My overall system isn't built to be cutting edge like that. I want great performance on current games and 8gigs of VRAM does not contribute to that goal right now.
Even if I felt like it might make life easier, it's going to be more expensive which is going to cut into the great price to performance balance of the 970.
I'd rather take my 4gig high end card, run the games on high at 60fps for the next few years then upgrade as the standards change than worry about future proofing. It's not uncommon that by the time your "future proofing" has a tangible impact, the new generation of hardware makes your future proofed gear obsolete.
Future proofing with PC gaming is so hard. It's very difficult to predict accurately how the trends are going to move and how quickly that will happen. Someone that spent a ton of extra money getting a 4 gig video card a few years ago when 2gigs was the standard never really saw that come to fruition. Now that games are looking for 4, the top end cards are $330 and blow those older cards out of the water.
