The ixbt link is a launch version (AMD had one manufacturer build all the launch cards, then the individual graphics card sellers put their sticker on it and box it as their own) while the first one you linked is a non-reference design (after a launch AMD and nVidia allow graphics card makers to make their own designs, and that's when you get differing cards with possible non-reference specs).
Your Radeon 5770 can be crossfired with a Radeon 5750, 5770, 6750, or 6770.
Since you're unsure about Crossfire, I feel I should remind you that you need a crossfire capable motherboard (one with two PCIe x16 graphics slots) if you want to run a crossfire setup.