i've google'd a little
there are a few issues so far - the macpro mostly fail on airport and video cards, some had ram issues
maybe you should start searching for "dust", check the fans and everything that looks like a cooling vent), remove the unneeded stuff from the mac, even the hdd and the optical drive (every card in it except for the video card and a set of rams) and try to start it again (if it starts, try it the next time, plug one of the unplugged items in and see if it still boots (and so on))
a few more info's
[http://arstechnica.com/civis/viewtopic.php?f=19&t=34597|here]
[http://discussions.apple.com/thread.jspa?messageID=3040563|here]
[http://macsaregreat.com/?p=63|here]