A couple of thoughts:-

1/ What else are you running when you try to run G2? Look into the two icon trays (especially the one on the bottom right) and see what is running from start up. Too many things running (including things like screen savers etc) and it will slow down any application you try to run.

2/ If you do replace the video card, make sure it's to an AGP port, and not the more common PCI slot, as the bus speed of an AGP is 3 or more times the speed of the PCI. The AGP is connected to the high speed memory management side of the chip set, and the PCI bus is part of the slow side.

My 2c (2p) worth!
-David C