I have dual G5 2 with 5.5GB ram and the other posts are correct. FCP, Soundtrack, and DVDSP are all great. Color, I tried a few times but way too slow. Motion is barely useable but I can still do some nice 2D stuff. 3D is out of the question. I am upgrading my graphics card so hopefully that will help me.
Personally, I don't think it is worth the upgrade to FCS 2, on your G5. Just a lot of cool stuff, you still cant do. But I'll see what happens with the new graphics card.