You can not store apps on the memory card. They're stored in what's called "phone memory" it's just non-volatile RAM. There may be an aftermarket app to allow you to store them on memory cards, I remember reading some comments but I don't remember.
Edit: if you don't want to lose your apps, don't worry. They're not stored on the card.
Aerize is an app which allows apps to be archived on the memory card. No problem switching out if you are only running apps and not storing them for some reason.
