Look at the Activity Monitor and see what is running. Look at the Page in/page out numbers after a day of use. Remember, having a billion files of stuff won't slow your Mac down. It only depends on what is running.
That's a valid point. Computers don't get "dirty." [size=-4](Well, they do, but a little time with some paper towels, a Mr Clean Magic Eraser, some canned air, and a Hoover takes care of that.)[/size] So if your Mac is running slowly, you should probably try to figure out why, instead of looking for some magic quick-fix program.
-Do you have enough free disk space (At least 15% or so)
-Do you have enough memory? (When running your normal suite of programs, at least, oh, 25% of your total memory should be "free" or "inactive" in the Activity Monitor. [EDIT: This is very likely a problem if you only have 512MB to start with]
-Are you running too many background applications? All of those cool add-ons like Growl, Quickilver, Shapeshifter and the like come at a cost in memory and CPU cycles. So do active Dashboard widgets.