Task Manager: XP on the left, Vista on the right.
On XP, I would check to see if Peak Commit Charge was approaching Total Physical Memory. When Peak exceeded Total during normal usage, the page file gets hit harder and performance degrades. I would know to recommend a RAM upgrade.
As Jeff explains, Vista uses RAM differently.
How do I read Task Manager in Vista to objectively determine if it's time to upgrade (i.e., the page file is getting taxed)?