My mac pro 1,1 is upgraded from the original dual woodcrest xeons 3.0 Ghz (2cores)
To dual Clovertown 3.0 Ghz (4 core). People do it and really it was not that hard to do. So now I have an 8 core machine.
Very true (got a 1,1 myself), but its also true that the later Mac Pro's had CPU's without their integrated heat spreaders (that's normally part of Intel's CPU) - making it a little more dicey for upgrades - since the one's you can buy have the heat spreaders and its affects the height with regards to the heat sinks...can be done but not as easily as the early Mac Pro's.
For a variety of reasons, Mac Pro's, while they could be upgraded, were never as easy as doing upgrades on a PC where you could just slap in whatever video card you wanted and whatever CPU that worked with the socket/MoBo...only a limited selection of video cards would work with OS X (since they needed Apple to write the driver for it) and the particular Mac Pro version you had.
I don't see it being a huge change from that standpoint (before you could do some limited hardware upgrades - basically taking it to the high end options later if you wanted)...JMHO... Beautiful machine, can't wait to try and hear one at a Apple store. ;-)