my 1 gig SD card was $20 on sale at circut city but you can find them almost everywhere - even drug stores carry them (many digital camera use them). the X5 only supports 1 gig or less SD cards so dont get anything bigger. CF cards can be any size but you should save the CF slot for other things. install your programs to the SD card instead of Main Memory (during the install it will ask you if you want to install to the Default location, tell it No and then select SD card).
grab TCPMP for your video player (free) and if you like chess head to
www.valil.com/chess for a very good free chess game.
I play movies, mp3s, read ebooks and play chess on mine, as well as keep contacts and notes. I also keep copies of some of my Word docs and Exel spreadsheets on mine but dont usually create them on the X5, just refer to them when Im out.
one thing you should do is get into the habit of making Full and PIM Backups, its simple, built in and fast. save them to your desktop box and/or SD card.