I'm going to guess you're not a designer..
Why, because I'm trying to give the OP some sound advice? I happen to do professional web and graphic design, amongst other things.
Now , of course you don't need an upgraded video card for design.. hell, if youre doing webdesign with some flash. and simple code.. why do you even need a Mac Pro? you could easily do that on a 3-4 year old machine and be fine..
The video card doesn't help with the current crop of design applications. He's not indicated that he's doing Final Cut stuff, Cinema 4D
et al., or photography work with Aperture - what else, besides games, uses the GPU in Mac OS X,
today?
Tomorrow?
Even if Snow Leopard makes it easy for Adobe to get their poop in a row and leverage the GPU(s), do you really think it'll be soon before CS5/6 comes out with that support? Do you really think it'd be hard to scrounge $400 for a new card when it might actually be useful?
For me, my 5+ year old machine works fantastically well with Photoshop and Illustrator. We have no idea where the OP is coming from. If I had a G4 tower, I'd be ready to murder no less than three motion video artists for an upgrade.
However, if a client wants you to do something BIG.. and pay you lots of money for it, you're going to wish you'd shelled out the extra cash while you had the chance to save a buck or two on it.
Even if the client wants something "BIG" from this designer, how is an upgraded video card going to help? Sure you can save $150 buying the fancy card when you order the Mac Pro, but if you don't need it, and the $200-300 upgrade price could be put towards RAM you
will need, how does that make sense?
It doesn't.
If you have two possible upgrades, you can only afford one, and only one is helpful which one do you buy?
Personally,
I would order the upgraded card in a Mac Pro, but then
I would actually use it with Aperture and video games.