Try ATI - they have the ATI Radeon 9800 Pro with 256 MB Vram and 2x 4x AGP capability. Also they have the 9700 Pro which is MAC /PC capable with 4x AGP and 256 MB Vram.
Buy.com has some great prices on these cards.
Since your comuter is a MDD model, Motion should load just fine. You do own it don't you? You know you can't just buy it by itself now?
I have the earlier ATI Radeon 9800 Pro Mac Edition with 128 MB Vram in my Quciksilver 2002 dual 1.0 GHx MAC to run Motion 2.