the software was pretty much designed to run on the paltry 64mb if you put 128 or 256 in there, be sure to up the integrated video memory allocation in the BIOS. I don't know if it helps or not, but when you got it, you might as well use it.
I popped out the 64 and put a 128 in there.. then use the 64 i took out in another machine to get it up to 128. I don't think i ever upgraded a force machine beyond 128. no real need to... if I did it was simply because i have a bagfull of ram sticks lying around.