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future proof is an urban myth m8 , unless your planning on not buying anything else in the future ?
just go with what you can afford at present.
In terms of PC gaming, future-proofing is nebulous at best. While DX11 will happen, will today's cards have enough grunt for DX11 games in 12 months time? Or maybe ATI will have refreshed their line-up and performance will have gone up and prices down?
Yes, if I was buying a card today I would buy an ATI 5850, but more from the cost/performance angle rather than it being 'future-proof' with DX11.
If you want longevity try to stretch to a 5850.
DX10 is old news already.
5870x2best as a more futureproof card