Should I download Google Desktop?

This depends upon the performance of the system you are using.

For example, the Basleine, or normal performance for my Vista box:

The performance of my Vista box after downloading Google Desktop:

From what I see the system is having to read more times from the disk and then as well the system memory is being used more by Google Desktops appearance.

For more info see http://desktop.google.com/features.html