The Quadro FX 4500 is the best card for running Motion and Aperture on your PowerMac. The 7800GT is a lot cheaper and comes close in performance, but you can't buy it without buying a PowerMac at the same time, yet.
I'd speculate that Apple will have a retail 7800 in a few months, and I think it may be one of the faster GTX or GTX 512MB chips instead of the GT.