What graphics card can i get which will be suitable for both OS X and MorphOS?
8500 or 9000 are the best (fastest) ones with 3D support in MorphOS.
These typically come with 64MB video memory, however, but for most things that should be enough. 92xx aren't as fast as 8500 or 9000 but typically have more video memory.
7000 series cards are slow and I wouldn't recommend them (unless if you can get 7500 for small money)