Swapping graphics cards

I have a G5 2 Ghz DP with a Radeon 9600 Mac Pro card with 64 Mbs VRAM and i work with FCP HD, After Effects 6 Pro and Motion. It struggles through RAM previews!
I have an older Power Mac G4 500 Mghz DP with an NVidia Ge Force TI 4600 card that I installed last year, to improve FCP playback. This G4 is now running ProTools and GarageBand in the 'music room', not doing any film work other than running QuickTimes, but this card has 128 Mbs of VRAM onboard.
My question is - can I swap these two cards over, get the benefit of DOUBLE the VRAM in the G5 for working with Motion and AE.
Will the Radeon 9500 Pro card from the G5 work in the G4 DP?
Will the Ge Force TI 4600 work happily in the G5?
I've learnt that interrupting a happily working machine is not always a good idea BUT I would really like to be able to work more efficiently in the two effects applications.

That's not a supported card for Motion and won't work:
[www.apple.com]

you would find stuff (other then Motion, which won't run on a 4600) running slower now as the GPU is MUCH more advanced in the 9600

DAVE at Movies Rock in Toronto

I was about to post again, to point out that in the After Effects list of approved OpenGL cards, the GeForce 4 Ti is listed as one of the best cards! But your post does now explain why it wouldn't be appropriate for Motion, and maybe other apps too. I have heard odd comments that AE is a bit "backward" and "uses ancient code" and such like, even though it's such a capable application.
Philip