Buy high range graphics card ATI?

Hi

With a budget of 15000 maximum, I would like to purchase a new ATI graphics card for intensive games. However, I am a bit lost between HD4870 512mb, HD4870 1gb, HD4870 2gb, 1gb and HD4890.

Could you kindly give me a good graphics card for the price I mentioned earlier (the possible with a test, I realize that the differences).

My motherboard is a Gigabyte GA-EP45-UD3R with a PCI-E 2.0.
Would you please advise me of the most powerful of these graphics cards, and take what brand?

Thank You

HD4870 1GB would be the perfect

Little interest to change a 8800 Ultra, while the DX11 compatible cards will get out of here shortly. Anyways, if you want to change immediately, then go for a HD 4870 512MB, ample for a 19. "1GB versions are not much for a screen this size, you need a 22" and + to see the difference.