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depending on the processor speed, speed/size of the harddrive, you may need more ram. The golden rule is that you can never had TOO much ram. I would try some things before upgrading, namily doing a few maintenance things, run a virus scan, spyware scan, clean disk, defrag, etc.
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All these are very good to do. You also just might have a lot of .exe's running on your machine taking up all your CPU usage.
There is a great tool called
Process Explorer. Its just like when you go into your task manager and see all your .exe running under the process tab. This program digs a little deeper and tells you exactly what each .exe running on your machine, including the ones start automatically start on booting windows. You be surprised how many unwanted .exe can run hiddenly without you ever knowing.