Well, just out of curiosity the other day, I added up all the percentages of memory used by the processes showing in Process viewer. The total came to about 126% :-)
But you have to remember that Mach is running the memory show here. Every process under Mach is allocated 4 GigaBytes (yes, GigaBytes) of memory address space. If you have 20 processes running, that's 80 GB of memory! So how much total memory in use isn't really that important; Mach will page stuff out to disk to make room for stuff that needs to be running in RAM.
But if you really want to know how much physical RAM is in use, open a terminal window and type in 'Top' at the command prompt. This will run the Unix program Top, which will show statistics on the top processes. There's a bunch of memory-related stuff there, including page-ins, page-outs, wired memory (memory that can't be paged to disk), etc. I think the last line will give you the total RAM in use. On my machine, which has 256 Meg installed, I'm usually using about 253 Meg or so of that if I have a dozen applications open. I really haven't noticed much performance difference regardless of how many processes are running, even when Bryce is rendering Animations in the background in Classic, or the Classic version of Setiathome is processing work units in the background.
Apple wasn't kidding when they said "out of memory" messages are out of here. :-)