What's the best graphics card available for any Mac? The 9400? Nothing released after 2006 will be able to run on them.
edit- Ok, the imacs run a 4850, which isn't horrible, but isn't great and won't run things at too high of res. And no the entire library won't be on there. 90% of the stuff out there is ran on Direct X. I'm pretty sure Valve still uses OpenGL, though.