These older Mac Pro's have gained still more value after the new Mac Pro was announced by Apple as many people like the internal storage options and have managed to upgrade the processors to what they need.
Can you give us the last three digits of your S/N (or Model Identifier) so we can ID which model you have and the OS-X version you are working with.
Have you checked your EFI & SMC firmware levels? Here's an Apple TN to help you do that for now hold off updating it [http://support.apple.com/kb/HT1237|EFI & SMC firmware updates for Intel-based Macs].
Are you finding a given HD or set of HD's are acting up? You may want to check which power line serves them.