its a best practice to install just about everything to external storage.
some programs that need to reside in memory, or today screen plugins, on the other hand, should be installed to the devices main storage.
examples like sbsh pocketbreeze, webis pocket informant, and lakeridge wisbar advance should be installed to main storage because of their reliance on system resources and sometimes programs can have hooks to built in programs that are essential for functionality, such as the calendar or contacts
maintenance programs, games, etc. should be installed to external storage. they will have a shortcut along with the other applications and games under the program menu.
most programs that need to be installed to main storage will say so during the install. otherwise, it may be mentioned in the readme or on the website from the developer.
be sure to read this before you do anything
Optimizing storage card speed | Smartphone & Pocket PC magazine
and this in regard to where you should/can install things
Managing My Internal Memory | Smartphone & Pocket PC magazine
use the search feature here on mobilitysite and the search feature over at smartphonemag.com for some of Werners' Complete tutorials and informations. he has spent years creating what is in my mind, the best resource for all things pocket pc and windows mobile.
jd