Currently, AE’s OpenGL implementation is not very good. It doesn’t work nearly as well as OpenGL in a real 3D app, and many users turn it off to avoid errors.
In short, don’t waste your money on a non-out-of-the-box graphics card (for AE) until the next rev of AE comes out.
And nobody knows when that will be (or is under NDA), so please don’t ask. 🙂
However, if you expect to run Apple’s Motion, get the ATI X800 or better. It actually uses OpenGL well with Motion, apparently. But don’t install the card’s new ROM upgrade if you have a dual 2.5 — according to the web, it’ll make your computer unuseable. Can’t speak for the 2.7.
Steve