You didn't say what operating system level you have. If you have OS X Lion 10.7, a number of people have had issues running Lion in 2GB of memory. 2GB is the minimum to run Lion, and 4GB seems to be the point where Lion's performance improves quite a bit.
Have you tried a safe boot, where you boot holding the option key down. This may take a little while, too. If performance is noticeably better, you should look at your login items (which aren't loaded in a safe boot) for something that is conflicting with the system. Ensure login items are the right level of program to run with your level of OS X. Ditto on the System Preferences pane, at the bottom, where additional software is listed.
You could try running OnyX from http://www.titanium.free.fr/download.php . Run the tasks on the cleaning and maintenance tabs. A number of people have found this helps with improving performance.
You can run the Apple hardware test to ensure there are no obvious hardware issues involved. See this: http://support.apple.com/kb/HT1509
This FAQ may be helpful too: http://www.thexlab.com/faqs/performance.html