[Cube] Influence of faster (or slower) graphics card>gaming?

[Stefan]

Hello friends of cube,

some time ago I upgraded to a 1.5 GHz GigaDesign 7457-Card and also a 
GeForce 4MX.

Encouraged, I tried to play some games, which were not playable 
before upgrading.
Really good was - for example - Ghost Recon.

But when I tested some other games I had to realize, that 1.5 GHz 
aren't enough.
Frame Rate is yet too low to play comfortable even in low resolution 
(1024x768 instead of 1280x1024)

My QUESTIONs are:

1) Which enfluence has a faster graphics card (faster than GeForce 4 
MX!) on gaming with my system
(OS-X 10.3.7 and 1.5 GB RAM, fast HD)?

2) Which (gaming) card can you recommend after own experiences?

Thanks for answers and tips in advance

Stefan