In the long term, I don't think DX11 support in the 5770 will matter much. By the time games begin to truly take advantage of DX11 for more than a few gimmicky features, the 5770 will be too slow anyway. It wasn't long ago that games started to take advantage of DX10, yet how many play games in DX10 mode on something like a GeForce 8600?
Also, I was never really bothered by the lack of PS3.0 support in my X800XT which I used for 3 years. PS 3.0 only mattered when you maxed out all settings, which wasn't possible on the 6800 either towards the end.
So I really don't think the 5770 is any more future proof than a 4870. The 5850 or 5870 are much more future proof, but currently slightly overpriced because of the lack of competition.
I'm also a student, and I'm definitely waiting until the 5850 falls below the $200 mark. But then I'm already using a 4850, which is quite a bit faster than the X1950.
Of course, if you have a Crossfire-capable board, you can buy one 5770 now and then just add another one later when they're even cheaper and games start to need the extra power.