Disagree. Things now move so fast that any excess power you currently have gpu-wise is exponentially falling in value by the day. Future-proofing is practically impossible and is financially nonsense these days.
Tomorrow's budget card will always match today's mid-range card, performance wise.
Think about the current situation with 5xxx cards. Why buy a mid to high range 5xxx when a lower group 6xxx would probably deliver similar performance. In reality that rationale holds true now pretty much all the time.
I guess the real question here is whether the OP is likely to upgrade his monitor very soon, say within in the next six months. If not then chances are that an adequate card today plus a further adequate card after the monitor upg would work out cheaper than buying a significantly over-powered card today and letting that excess power do nothing but fall in value for half a year.
Cheers,
vfm
I
It works out the same to be honest.

What if a 460 only gives him 2 years of playable performance but a 5870/480 gives him 4 years? Sure he pays £150 extra now but then, he wont have to buy another for 4 years, where as in 2 years time he might have to buy another mid range card at £150.
It's all swings and roundabouts to be honest.
In terms of his question though, if you plan to keep the 460 for a very long time then get the 1gb version ( also leaves room for a monitor upgrade)
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. Attempting to 'futureproof' in such a fast moving area as PC hardware is a mug's game.