Benchmark for Linux

I work in a computer company which sells computer configurations and parts of them. And I want to give a choice to customers. If they want to buy a PC with Linux installed, not Windows. But I find difficult to test the Graphic Cards in Linux OS. I have searched the web and I didn't found any benchmark program working on Linux. And I need to test every Graphic Card before it is selling. Any sollution?
Regards, Vlatko

Do you know about this organization?

http://www.spec.org/

I didn't knew. I hope I will solve my problems.
Thanks very much.