In windows task manager my VMware always says 512megs which is what i issue it, i think it just saturates whatever you give it.
Thinking about this the answer is obvious, test it!
I've just booted my ubuntu with an allowance of 450megs through VMware, timed it doing three frames then rebooted ubuntu this time with 1.5gigs of memory (which VMware took all of in windows task manager, but ubuntu still said it was only using 350ish megs) and timed it for 3 frames again. The times were both virtually identical so i think any memory over and above 450megs is wasted if you are purely folding in ubuntu on VMware. Vmware will just eat the rest of the memory you issue it slowing down any work you try to do in windows.
Obviously if there is a WU out there that requires more memory than the ones i am currently getting then it may need slightly more memory, 512meg is still more than enough IMHO though.
Perhaps someone else would like to do the same test and see if their version of virtualised ubuntu benefits from being issued extra memory? I am running 1 SMP client on a single virtual machine on a dualcore intel.