The 5970 is a 5950 really, a 5970 will become a 5950 in reality and the new overclocked versions(or stock clocked 5870 stylie ones) with more memory are basically the real 5970.
Likewise a 5890, or overclocked 5870's, are nothing you couldn't achieve since launch with a 5870(and pretty much a 5850) anyway.
A name doesn't change performance, and a 5890 that can do 1ghz/1.3ghz, or a 5870 that can do 1ghz/1.3ghz mem, will perform the same, though a knock on effect of prices is the only interesting part.
As for playing Metro 2033 in full, its a very short game and one offs with zero replay value really don't require 90fps solid to enjoy. LIkewise turn of DOF and tesselation and a stock 5850 can give you well over 40fps average in a game that isn't a uber fast paced fps and feels smooth at 30fps.
Would you keep on tesselation and DOF if the effects were renamed to "does nothing " and "jokes on you" ?
No, would you turn them on if they actively decreased IQ while lowering performance? No, then why must you have them on when they offer no IQ increase and half performance, just so you can say you maxed it out? I maxed the game out, while turning those two options off, and its more than playable on a £200 card I got on launch the best part of 6 months ago.