The best cards for FCP X are based around the AMD GPUs. They will provide the best acceleration for you.
The problem is finding a low profile card that's passively cooled. I'm not aware of any but I'd keep looking. The nVidia cards are not good matches for FCP X do avoid them. Also you will need to disable the internal graphics card to get the acceleration in FCP X.
Unless you really really really need low profile passively cooled then look for 5770 and upwards
7970 and or R280x