Firstly AMD and NVidia both have a range of cards with different power levels. Getting a card with 8G that uses the 75W available from the PCIe slot should be possible.
A card that is setup for ML isn't necessarily going to work well for desktop use. I currently have a NVIDIA card in my workstation setup for ML and it gives 0.2fps on games. I installed the NVIDIA card because the AMD card died and decided to play with ML on it.
I think the best thing to do is to use a separate PC for ML.
