OK,
I'm going to throw a bit more confiusion in here for andy... Get the ATI card! Don't look at it as if it's a gaming card, it's more than that. It's only a $250 upgrade and is the best performing card in all aspects, even for 2D functions, of all the current choices with the exception of the overpriced FX4500 -- which only has advantages for specialized 3D functions.
The X1900XT is the best bang for the buck right now on the Mac Pro and will accelerate lots of Quartz functions as people have stated. It will really show a huge advantage over the 7300GT offerings with Core Animation functions coming up in Leopard (OS X 10.5).
OK, enough on that... As for the Photoshop benchmarks and have to side with iGary. Those benchmarks are pretty close to correct from what I've seen in "real world" use. In fact, with many operations and especially if you have to resort to 3rd-party plug-ins, my G5 quads are definitely faster than a Mac Pro.
OTOH, the Mac Pro is still blisteringly fast considering Photoshop is running via Rosetta. I can hardly wait until we have proper universal binaries for Photoshop. Especially if CS3 is properly multithreaded to take advantage of all these CPU cores. It's going to scream....