Is upgrading the Mac's video card worthwhile?

Can anyone tell me if a better video card can make a significant difference in the performance of Final Cut Studio? I'm setting up a G5 Mac to use for mainly video editing (with Final Cut 5.1 under OS 10.5.8).

I have acquired a G5 Quad-core 2.5 that I am maxing out with 16 GB of RAM, and it will have a couple of 2-gig 7200 hard drives in it. I'm wondering if its stock video card, a NVIDIA GeForce 6600 with 256 MB of VRAM is adequate, or whether I would be better off replacing it with a Nvidia Quadro FX4500 with 512MB.

I'm thinking that if I could gain a noticeable performance boost with the better video card, I would go ahead and spend the $150 or so that these cards are going for nowadays. But if the improvement would be minor, I won't bother.

Anyone here have any experience with upgrading video cards in a G5?

Tom