Q: memory

How much RAM do you have? Go to the Apple menu and choose "About this Mac" and see the amount listed there. For Lion you need at least 2GB of RAM, but will do best with at least 4GB. If you are not sure how to upgrade your memory then take your system in for servicing; otherwise you can try your hand at purchasing a memory upgrade and installing it yourself. This is one user-serviceable aspect of the system--see here: http://support.apple.com/kb/HT1423

Also review the CPU requirements for Lion.  The issue of memory might be moot.

Have a look here for cheap but reliable memory upgrades:

http://www.crucial.com/mac/index.aspx

Be sure that memory isn't the only problem. Make sure you've checked the rest of the h/w specs here:

http://support.apple.com/kb/HT4949