Hello & Welcome.
No it doesn't worth it, HD 5970 failed in some reviews to perform because of the high temps and noise, so picking up a used HD 5970 is not that great idea.
If you want to play Star wars with playable frames @ highest settings @ 1080P so you can pick up a HD 6970, or if you wanna invest your $400 pick up 2 HD 6950 and CF them for the ultimate gaming performance.
I'm with refillable- a 5970 seems like a great value proposition (overall it should perform just above a GTX 580). For some people it may be. However, for most folks that want to keep their hardware up to date a 6970 (or a GTX 570) is a better decision. Provides an upgrade path!
Do you think that it will be a wise move to wait for ati series 7XXX or you think that the difference from HD6970 or GTX570 will be minor on this price range?
I know there aren't many clues yet but maybe someone knows better and enlighten me.
HD 7950 is said to be $450 and will be released Q1/2 2012, it's equipped with the new XDR2 rambus memory which is twice faster than GDDR5 besides the 1 GHz GPU frequency and 28nm process for low power consumption and less heat.
The HD 7970 is said to be $550 but no more info about it yet, both cards are equipped with 2 GB of VRAM.
HD 78XX will be probably pushed in next Jan, but they will be a minor upgrade over the current HD 69xx.