Our friend Barefeats.com has published a 3D games comparison between different Mac Pro and PMG5 Quad models. It clearly shows that depending on quality settings, the Mac Pro can compete despite its Geforce 7300 with a PG5 Quad equipped Geforce 7800 GT; demonstrating that some games are more CPU-depend than others.
For real gamers, requiring higher quality settings, one will definitely want to install a Radeon X1900 XT, in order to give real GPU power to a Mac Pro.
To read the test:
http://www.barefeats.com