Lisa, one of my favorite on line stores is
www.Newegg.com. Very reputable and safe. Tiger Direct isn't bad, but in my opinion not as good as Egg.
I put almost all my apps on a card. Basically I put all apps on my 1G sd, and all movies/mp3s on my 4G cf. I store very little on main memory. Read the tutorials, there's an excellent one on memory.
Look at getting a backup program like Sprite, sunnysoft, and spb has a trial version (I think). Backing up is very important.
You can't put an app on a card then move it. You have to uninstall then reinstall. Certain few apps run best in main memory, the other 99% can go on a storage card. CF cards are cheaper than SD cards.
Hope that gets you started.