AndyFC is correct.
Using a vanilla 3.1 install, its about 7.7mb left after loading workbench.
3.2 loads stuff into RAM, if you do not have 3.2 / 3.2.1 roms, then it will load extra modules into RAM.
Easy way to check, is early startup and boot with no startup-sequence and use avail to see how much FastRAM you have, like AndyFC said.
Brad if you need any further help, give me a call.
My current test system is running custom 3.2.1 roms and OS 3.2.1 and consumes around 4.4MB's of my 64MB's of RAM.