i say wait till 8800GT is back in stock. I see your mobo supports crossfire but the second pci-e slot is x4 speed... whereas an X38 mobo has x16 speed on both slots fully utilizing crossfires power.
Get the 8800GT enjoy it and when you get to the time you feel you might need Sli (2x8800GT).. think about getting a new mobo. You can simply transfer your Q6600 cpu and graphics card, ram and other stuff over.
plus 8800GT is like an asset. if you got 1 or 2 youll always be able to sell them easy whenever you like.
unless someone says that crossfire with the second pci-e slot at x4 doesnt matter thats whats i would do. Just an idea