I would go with a Nvidia graphics card Dom , I've found that Nvidia support for linux drivers better than ATI, they are more uptodate and work out of the box on most systems.

As an example I have an old GeForce 4, I upgraded to the latest Drivers and I now have a machine that supports opengl 2.0 api, even though some of the api is not supported in hardware by the graphics card the driver can still emulate it. I very much doubt you'd get that kind of service from ATI..but I could be wrong.

As for CPU's / Boards, not sure, my kit is quite old so I'm way out of date on that stuff.