The whole concept of an app "using memory" is a whole lot less significant these days under Mac OS X than under Mac OS 9.
In Mac OS 9 everything was doled out in very brittle heaps when an app was launched. You had to worry about fragmentation of the memory space and how your app would fit all of the code and the user's documents into the RAM space.
Under Mac OS X that's largely irrelevent for the end user. The industrial quality virtual memory system of Mac OS X means that your app asks for what it needs. The only parts that are "wired" to real RAM are in the kernel so unless you're writing device drivers this is largely irrelevent.
That's not completely true, an Apple developer on the OmniGroup developer mailing list said that the kernel hard wires around 17 megs of RAM. If you are trying to squeeze apps and data into memory you'll be much happier if it all squishes into as much "real RAM" as you have. No app can "hog" your RAM just by having it open (like in Mac OS 9) but if you have 160 megs of RAM you'll get much swifter performance than with 64.
I'm curious what you want to do with these RAM numbers? If you're a programmer you probably want to have a second machine set up and get your stats via ethernet so that the top command itself doesn't spoil the numbers too much. If your an end user, you can get some numbers but they'll be highly subject to what you're trying to do at the moment and the only real control you have as a user is whether to plunk your VISA card down and spring for more chips or not. I dunno about you, but I find that much easier to do by "feel" rather than with top.
