I just did this last night because I also took delivery of a new Mac Pro which had OS X Lion installed.
Fortunately, I had a Snow Leopard Drop-in DVD (version 10.6.0 - yes 10.6.0). Here is what I did:
1. Booted from SL Drop-in DVD by holding option after a restart
2. Went into Disk Utility and added a 2nd partition to the existing Macintosh HD partition. I did this because I wasn't sure if this Drop-in DVD would support the ATI 5770 since this together with the Mac Pro was launched with SL version 10.6.4. I also wanted to keep the oS X Lion emergency partition on there just in case this Drop-in DVD did not work.
3. I was able to successfully install it with no problems - I had a 30" HP ZR30W monitor and the video card and resolution were fine. Also, as soon as I booted I went into System Report to verify everything. All the devices were reporting no problems. The only hiccup was that in System Preferences under Display, there were no calibration profiles.
4. I immediately updated to 10.6.8 via Software Update. After this, the calibration profiles were now listed in System Preferences under Display.
5. Everything has been fine and the Mac Pro is operating great.
I do not want to sound arrogant, but I have to use Lion on a MacBook Air and while there U.I. enhancements are nice, I really loathe the overall Lion experience compared to Snow Leopard. Even if it is just for the iCal and Address Book interface. Not to continue hi-jacking my own post, but I simply do not see why Apple did this. Even the removal of iSync was a mystery, I relied on this heavily as a non-smart phone Nokia carrying person.
Anyways, I digress... I have an extra Snow Leopard DVD Drop-n here at my house. If you would like it email me here: michael710 at me dot com
Best,
Desmo