It really depends on what resolution you plan on playing on over the next few years. It also depends on what you consider to be a "max out" of a game and what "decent framerates" are in your standards. If you like antialiasing, 2GB will is already too little, especially on higher resolutions. As an example, I play on a 2560x1440 monitor and I play with AA maxed out, so I'm using over 2gb of vram for most of the newer games out there. Hitman Absolution sometimes uses over 3gb of vram. I would recommend at least 4gb of vram for a safe future-proofing setup, although nothing is truly future-proof given the uncertainty of technological progress and upcoming requirements.
From a raw power standpoint, the 690 will not gice you 60+fps at higher resolutions for the most demanding games, so you would need 2 eventually and after that, your only option is upgrading to new cards because you'd be in 4-way SLI. The Titan is quite pricey, but it offers the most future-proof at the moment. 2 Titans will run anything with maxed AA, although Tomb Raider and Crysis 3 put up a bit of a challenge.
In summary, if you want to truly max games out for the next 4 years, no single card will do that at 60fps and especially not at higher resolutions. Your best bet right now is 4 Titans, but that's $4k and for all we know, the Maxwell architecture could yield a flagship that's more powerful and cheaper than a Titan, so you wouldn't be at the top anymore. Unless you're willing to upgrade every year, just get the best you can now anf enjoy maxing games out while you can.
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