Re: Which video card would be better?
CrossfireX (HD 3800 series onwards) supports upto four GPUs. Up to four GPUs, HD 4870 X2 already has two GPUs, so it only works with two cards. You can run four-way HD 4850 CF though.
As for HD 4870 X2 vs GTX 280 SLi, it isn't in the same price range is it? But I would say the HD 4870 X2 mostly because you wouldn't need an Nvidia motherboard, and because a single X2 is actually faster than two 280's in more than a few games.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.