Does anyone know if you can store apps on your memory card, to free up space on the phone, or will they run too slowly, or be a PITA to access?
Until someone get a working custom kernel for this phone you won't be able to use data2ext (which would move your entire data partition). However in the meantime there are two alternatives. Links2SD might work though I'm not certain it may in fact require a custom kernel as well. The simplest solution is to actually use the built in froyo options to move at least some apps to the sd, you could also use App2SD from the market to do this automatically for you.
The main issue with apps stored on the sd card is that at least sometimes shortcuts get lost on reboot and similar, moving the app back to the phone usually resolves those issues. Since the prevail doesn't have a lot of customization options yet I don't really think you will have to many options with that.
Some apps placed on the sd card will of course run slower than others simply do to the read rate of the sd card and similar.
In short, yes you can move apps to the sd card but if you have issues with a specific app, move it back to the phone