It sounds like you've already got a tool to visualize this.
Add the "private bytes" column to the process explorer display, then sortby it descending. Your memory hog processes will be at the top.
Also you could look at "Resource monitor" in Vista. (System and Maintenance > Performance Information and Tools > Advanced Tools. It will also show you which processes are using all your memory.
However, you may not really even have a problem. RAM being in-use is not a bad thing. For windows to use all of that expensive (used-to-be, anyway) RAM that you installed, it tries to, well, use it (as in utilize it). If you aren't using all of it for programs it will use more for the system cache to possibly save time reading from the disk in the future.
Rather than how much is in-use, you want to worry about available memory.
To quote Mark Russinovich:
As its name implies, available memory
is available for assignment to a
process or the system if required. The
Memory Manager of course tries to make
the most of that memory by using it as
a file cache (the standby list), as
well as for zeroed memory (the zero
page list), and Vista's Superfetch
feature prefetches data and code into
the standby list and prioritizes it to
favor data and code likely to be used
in the near future.
If available memory becomes scarce,
that means that processes or the
system are actively using physical
memory, and if it remains close to
zero over extended periods of time,
you can probably benefit by adding
If you have "100 MB" you are not close to zero free memory.
Oh, and make sure you use a paging file, so that windows is free to help you make the most of your RAM.