I have resolved my MacPro sleep issue by removing the Seagate 320GB stock drive.
For whatever reason, if that drive is plugged in, it reboots my MacPro when waking from sleep.
I had setup 2 WD 500GB drives in RAID 0 as my boot volume and this was configured with Windows XP Pro for other uses.
Hope this helps someone else.
Cheers!
Haven't had a reboot once. I took out the stock 320 GB drive from the beginning (7,200.10 drive btw - how cheap of APPLE !!) and I don't have the 8800 GT - have the FX5600 and RAID card with 4 1TB drives. Never heard the fan rev up - the machine is quieter than my liquid cooled VooDoo PC !!!