Leopard can run both 64 and 32 bit apps, and the mac pro can allocate any 32 bit app a full 4 gig of memory
The next version of OSX, Snow Leopard will see the entire OS move over to 64 bit, for more info see here
Road to Mac OS X 10.6 Snow Leopard: 64-Bits — RoughlyDrafted Magazine
It looks like Maya on OS X is only 32 bit, but at least using Leopard it gets a full 4 gig of memory