I only upgrade if I get a minimum of 50% performance increase. My upgrades were:
X2-4800+ and HD3870
i7-860 and HD5870
i7-3770K and GTX680
My next upgrade will be a GTX870/880 likely (Maxwell architecture) with 4GB of VRAM. I may even get one with the Denver ARM CPU which rumor has it may benefit for PhysX and other tasks though there's simply no solid info on that.
2GB vs 4GB:
4GB on a GTX770 is currently a waste of money. Some would argue that SKYRIM can benefit with mods, but then the performance drops way below 60FPS so in that case I'd be recommending a better card anyway.
MANTLE:
Not many games, but some are coming and the HD7970 might perform better in those than a GTX770 especially if you have a weaker CPU such as an FX-6300.