On eBay I can get two R9 270Xs for $200. Is it worth it? I game, use blender, use gimp, use kdenlive, and run Korora Linux.
Pretty anything AMD will work on Linux. I believe they are going to "free" their Radeon drivers soon so that'll make it even easier for them to work in Linux.
Crossfire is not something that would work well in Linux, but a single 270X works pretty well with the open source drivers.