Ehrrr.....calculator is the last thing you want to worry in PPC.
Simple symbolic calculator
http://www.saltire.com/samples.html
Scheme
http://www.mazama.net/scheme/pscheme.htm
Maxima for CE. (warning, this industrial strength software is BIG, takes 2 minutes just to load the engine)
http://www.rainer-keuchel.de/wince/maxima-ce.html
There are also HP48G emulator, and TI-89 port is on the progress. (search the net)
plus there are also the usual mickey mouse calculator like you've just mention, some are quite big and elaborate.