Finally! :-)
Just as I suspected. For the workload your PC is normally under, you simply do not have enough RAM. If you were to double the amount, your performance would surely improve.
If you go to http://www.crucial.com/ and run the Memory Advisor Tool, you will get a good match. Post a link to the results if you think you need guidance.
But...
I still need you to post the CPU usage (percentage) figures! This image should help you understand what I am asking for:

http://whall.org/blog/files/perf-cpu.jpg
As you can see, the top three processes and their corresponding CPU percentages are:
System Idle Process 92
iexplore.exe 11
SnagIt32.exe 03
What are your top seven?
AND
Please repeat the process for Mem Usage whenever your PC seems sluggish.
Last question for now:
Is your PC its worst when you first turn it on? (Like for the first ten minutes or so?)