The sticky threads at the top of the 8100 section of this forum will have all the info you need on upgrading your OS.
For memory, frequent battery pulls will help restore memory. Also be sure to close applications when they're not in use (Menu > Close/Exit). Using the red end call key doesn't close them. It leaves them running in the back ground.
Check some of these links to help with freeing up memory.
http://www.blackberryforums.com/gene...ee-memory.html
Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ
How do I free up memory on my device? - BlackBerryFAQ