Using the performance tab in Windows Task Manager, how can I tell if I need to add memory to my PC?

I'm tuning up my PC and I'm wondering if I need more memory and I'm hoping the performance tab in the Windows Task Manager can tell me. How do I read the Physical Memory, Kernel Memory and Commit Charge information to determine if I need to add memory?

Thanks!
Update: I'm just trying to figure out how to interpret the data on the Performance Tab in the XP Task Manager. I'm already familiar with the generic ballpark guidelines, but I'm looking for someone who knows how to interpret the data in task manager.

Let me know if anyone has this knowledge. Thanks!
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