What about Quad Core upgrade by yourself?
"...you could theoretically drop two Clovertown
processors in there and you'd have an 8-core Mac Pro...
...We grabbed a pair of 2.4GHz Clovertown samples and
tossed them in the system, and to our pleasure, they
worked just fine. Our samples used a 1066MHz FSB,
although we're expecting the final chip to use a
1333MHz FSB, but the most important part of the test
is that all 8 cores were detected and functional..."