2006 Mac Pro Processor Upgrade

You can upgrade to the other Clovertown processors.

Not Harpertown, and not Gainestown.

You'd need to buy the processors... and probably thermal paste.

 

it is possible to upgrade to the first quad core xeon's that intel released (clovertown), but not penryn or later. Of course you could go for faster woodcrest (dual core) chips, but it's probably not worth the time or money

edit - as tallest says, the Harpertown chips don't work, it was the product code name for the first penryn based xeons... basically, any of the 51XX or 53XX series with a FSB of 1333 should work I believe