As for a new card the 5870 is about as fast. Thats $400. The GTX470 is probably about 70-80% as fast as well and should be around $350.
I would OC an HD5850 or GTX 470(it's actually $300 these days) instead of getting an HD5870. They are much better deals for them money. The HD5850 on particular is much cooler and more power efficient than the HD4870x2.