Clock speed and memory speed aren't the only things to take into account.
The best thing to do in your position is to look up benchmarks. Say you're thinking of getting an HD5770, go on Google and search for 'HD5770 anandtech' and look at the benchmarks against other cards to see how it performs.
You can't just look at one spec and determine if the card will be faster/slow than another.
Thanks for your advice. However it'll take 1 or 2 months the 'buy time' to come. mkey82 recommended me some graphics cards that can run the game and its coming version too.