Well, do be honest i have the 512MB memory card the phone came with, and thats primarily used for storage. Running programs are based on the Memory of the phone itself. It just depends, if you have Internet, media player, text messaging, bluetooth, contacts, etc all running, you'll be low on running program memory. With me, i have 94 MB's of Program memory. the more programs you have open, the less memory you'll have available. Erasing the Internet history files and cookies help ALOT. Also, soft resetting the phone every once in a while help in always having available memory! If your an avid texter, and your running low on program memory, i know you have to text real slow as it takes forever to input your text. But its not the case on Micros...
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The memory card is just for storage. It can't be used to add to the memory/ram of the device.