Which graphics card to buy?

I am confused about which graphics card to buy, its either Sapphire Radeon HD5750 1GB DDR5 VAPOR-X or 2X (Sapphire Radeon HD5550 1GB DDR3) connected via crossfire, and my motherboard supports 2 PCI-e express card slots, but i dont care that my 2 slots will be lost, but is it true that ill have 2GB memory if i crossfire then, and double speed!, oh and can a microATX motherboard fit in a ATX case perfectly?thanks anyways