Unlike WM2003SE you cannot change the RAM settings. In WM2003SE there was program storage space allocated in RAM, and you could define how much RAM to use for storage and how much for runtime. This is not the case in WM5; RAM is only used for running programs and ROM is used for program storage.
One of the problems with WM5 (IMHO) is that the OS uses way to much of the RAM for itself, and didn't leave enough for runtime.
To answer your other question, no you cannot use SD or CF card to expand RAM.
Maybe you'd be interested in upgrading RAM from 64MB to 128MB? Try this link for more information.
Upgrade RAM