Finally! I can use Leopard. I bought it back when Apple released it. The one and only thing that kept me from using Leopard up until now was a problem with the system freezing randomly. It appeared to be something that involved my ATI X1900XT that came in my Mac Pro from the factory. Well, now that 10.5.2 was out and I also noticed a core graphics update had been released, I decided to try Leopard on my system again. Even did an upgrade this time (after backing up of course!) instead of a clean install. Guess what..... ZERO PROBLEMS!
Needless to say I am very happy.