Run Verify/Repair Disk.
You should have at least 15% free space available on your HD; if not, need to delete some things.
You may need more RAM. One way to get an idea whether more RAM will help or not, is to check Page outs and Swap used. First, use your Mac as you normally would, the longer the better (checking this right after starting up isn’t likely to show you anything). After you’ve used your Mac for awhile (again, the longer the better), go to Applications > Utilities > open Activity Monitor > in the top menu at right, select All Processes in the drop down menu > near the bottom of the window click on System Memory. If Page outs is more that about 10% of Page ins, and/or Swap used is large, chances are good that adding RAM will help. http://support.apple.com/kb/HT1342