PIX is basically the same thing as ASA, but it doesn't support the newer code. The major code changed when PIX 6.3 turned into ASA/PIX 7.0. If you can get your PIX 501 to run any code that is 7.0 or later you'll have a pretty good feel for how the ASA works.
Is it possible to put 7.0 on a 501, and if so where would I get a copy? and how much?
edit: Ok I see here http://www.cisco.com/cisco/software/release.html?mdfid=270664372&softwareid=280786991&release=184.108.40.206&relind=AVAILABLE&rellifecycle=&reltype=all That I can go as far as 8.04.28, but to get a copy I need a service contract, kind of hard for someone studying at home.