a friend has a 4830 and he didn't told me about any problems
if it is a x2 i don't know how it works in linux but the driver at least should enable one of the gpus, about the size of vram, it shouldn't be a problem
"I am asking because I know ATI support can be ify."
afik, that is not the case anymore
remember to install video driver, there is some posts mentioning how to do it