I think you are mistaken about he 64 bittedness of macs.
Certainly ALL Mac Pro’s since 2008 use the 64 bit kernel by default. (see the list below from the Apple site).
As you know the main thing about 64 bit applications is that they can access much more memory, so much more complex files can be created. So put as ram as you can afford into your mac.
I am using the 64 bit version of Carrara on my iMac and it is working well with OSX Lion.
Having said all this it is important to note that even if you cannot or do not boot kernel into 64-bit mode, you can run your 64-bit apps as 64-bit, and they can take advantage of all the memory in the machine.
You can see which kernel you are using in System Profiler:
Choose About This Mac from the Apple () menu.
Click More Info.
Select Software in the Contents pane.
Look for “64-bit Kernel and Extensions: Yes (or No)” under the System Software Overview heading.
If your Mac uses the 32-bit kernel by default, but supports the 64-bit kernel, you can start up using the 64-bit kernel by holding the 6 and 4 keys during startup.
These Macs use the 64-bit kernel by default in Mac OS X v10.6.
Mac Pro (Mid 2010)
MacBook Pro (Early 2011)
iMac (21.5-inch and 27-inch, Mid 2011)
These Macs use the 64-bit kernel by default in Mac OS X Server v10.6 (they can also use the 64-bit kernel in Mac OS X v10.6, but do not use it by default).
Xserve (Early 2008) and later
Mac Pro (Early 2008) and later
Mac mini (Mid 2010)
These Macs support the 64-bit kernel, but do not use it by default.
iMac (Early 2008) and later
MacBook Pro (Early 2008 through Mid 2010)