funguseater :
Hi
First off, if you are building a gaming PC then forget about socket 2011, it performs on par with 1155 for gaming at a fraction of the price. Plus there really isn't such a thing as future proofing, intel will be releasing a new socket for Haswell and AMD has pushed back the release of Steamroller to sometime in the future.
If you can wait until next summer (I know that's forever in the tech world) Haswell will be released and if rumors are correct it will be the last CPU type processor before everything becomes integrated into APU's (lame).
Also next spring AMD is releasing the new 8000 series GPU's which are said to use HALF of the wattage and provide almost double the performance (yeah well we can hope right)
If you were to build a system now you could build an i7 3770K with a $150 z77 motherboard and 8 GB Ram pair it with a HD7970 GHZ ED and a couple of SSD in RAID you really cannot get much faster than that.
Have Fun
Fungi
I'd have to disagree with what you say on future proofing.
Sure, 5, 10 years ago it was a mad idea, but considering the stagnation of gaming advances due to the limitations of their static hardware, and how PC hardware has left the consoles in the dust, I think it's more then reasonable to make a future proof/next-gen PC if you are smart about it.
We already know a fair amount about the desired specs of the next gen of consoles:
http://www.joystiq.com/2012/03/09/battlefield-3s-rendering-architect-details-his-dream-specs-for/
http://www.geforce.com/whats-new/articles/stunning-videos-show-unreal-engine-4s-next-gen-gtx-680-powered-real-time-graphics/
http://www.tomshardware.com/news/Kepler-Nvidia-Tim-Sweeney-Unreal-Engine-UE4,15571.html
So if we take console optimisation into consideration, as well a power constraints, I think it's safe to say that the next-gen consoles (not the wii u - we know the specs of that are dire) will use about 4 - 8gb ram and I'm going to guess have the power of somewhere between a GTX560ti and 680 if we are lucky. Maybe they pull a trick out of their pocket and give us a GTX680 but that would be surprising to say the least.
So from the above, if you get yourself a motherboard that can take up to 32gb ram and SLI, has a CPU that won't bottleneck two GTX 680's and for the meantime just stick with one GTX 680, I fail to see how such a PC wouldn't be future proof while also not costing an extortionate amount.
If I'm not mistaken an OC'd 9950 with 7900 GT in SLI would still currently be better then both the 360 and PS3 if we consider the consoles gaming resolutions. Nowadays the power draw of the bigger cards is far beyond the power draw for consoles so if you go for a power hungry GTX680, I can't see how the next gen would/could supersede that GPU, let alone 2.