well i have mac pro, too. it's one of the older models, but either way one year ago i also had to chose between the ATI x1900XT and the Geforce 8800 GT. in that time i read that the Geforce is better for gaming performance whereas the ATI is suposed to have better 3D attributes, whatever that means.

while rendering, you mostly won't notice any difference to which graphics card you use, since the rendering will only challenge your CPUs.
you WILL notice the difference, if you're working with hires models, especially in the shaded node because that's where your card comes in. but since maya (even 2010) only arrives in 32bit for mac, it'll be wiser to buy a windows computer if you work a lot on such hires models (but of course that's not really an option. its windows.. )

so in this article https://www.evga.com/articles/00479/ which of course isn't very objective the producer reclaims this card to be suitable for 3D modelling, too. i know that the ATI is bought a lot for film producers since it has a HDMI but that's not relevant for you.

but what is relevant is MAC OSX 10.6 which is sold next week! as you surely know, this OS will work a lot with openGL, so other apps may use some power of your graphics card and then i'm pretty sure the Geforce will be a lot better then the ATI.

Phew, sorry for my lame english, but i hope you get the point. i didn't find any benchmarks for your new card, but i think with that 100$+ you make a investment for your future. that won't hurt and you will profit when using other apps as maya ,too.

solong