Want to increase spead and reponsiveness in older Mac
Upgrading to Lion or Mountain Lion would not improve your Mac's performance. It may actually decrease performance as the newer versions of Mac OS X seem to require/want more RAM to run smoothly. Apple has increased the minimum amount of RAM in almost all of their line up to deal with this issue.

You currently only have 1GB of RAM... upgrading this would probably be the cheapest and most efficient method of speeding up your Mac. Freezing and unresponsiveness can be caused by lack of RAM. 

Check out this Apple support article that will teach you how to use OS X's Activity Monitor to check whether RAM would improve performance.

After determining whether or not you need more RAM (you do!), you can find out what kind by following these steps:


Click the  in the Menu Bar
Click "About This Mac"
Click "More Info..." in the About This Mac dialog box
Click on the "Memory" tab


Listed will be what type of memory you have and how much. In the lower right hand corner is a link to "Memory Upgrade Instructions" — follow these instructions.