1. there is some getting started in AMD APP SDK documentation. also I recomend read AMD OpenCL programing guide also in AMD APP SDK documentation.
2. I think the X variants off the cards are not worth it. buy 280 or 290 instead. but for start any 5xxx or newer card is fine.
3. yes. But you may consider run your code on CPU first as if you lock up your GPU with while(1) loop you must hard reset whole PC.
4. yes. each GPU is exposed as one OpenCL device so you create command queue for it. so which queue you use determine which GPU will execute kernel. crossfire is nothing to do with OpenCL. in fact sometimes you need disable it to access multiple GPU.