Which of these graphics cards should I get?

1. XFX Radeon HD 5770 1 GB DDR5 PCIE Graphics Card
2. XFX ATI Radeon HD6870 1 GB DDR5 2DVI/HDMI/2x Mini DisplayPort PCI-Express Video Card
3. XFX AMD Radeon HD 6870 900M 1 GB DDR5 DUAL MINIDP HDMI DUAL DVI PCI-E Video Card
I have a Dell XPS 8300, iCore 7, 3.4 GHz processor, 12 GB RAM, 1 TB HDD, Windows 7 Professional.
Answer #1
2
Answer #2
What is the difference between 2 and 3? I would say one of those
Answer #3
With something like an I7, it would be a waste if you don’t get at least a GTX 560 Ti or a HD 6950.
Besides the cooling of the GPU heatsink, and the amount of memory, there isn’t much performance difference between brands.