Having problems booting Valve games with a HD 6570.

I just recently bought a XFX Radeon HD 6570, for my Mac Pro, and everything seems to be working great. My only issue is that I can't seem to open any games made by Valve. I tried TF2, Portal, And Left 4 Dead. I've been searching everywhere for an answer, and I'm coming up dry.

Here are my specs:

Mac Pro 1.1
Running OS X Lion (10.7.5)
2x 3.0GHz Dual Core Xeon processors.
3 GB of RAM
Radeon HD 6570 Core Edition, 2 GB DDR3

Thank you!