The Desire doesn't run at 1.5GHZ unless over clocked, it's a 1ghz processor, mine usually ran at 998mhz.
About rooting the phone.. I *think* the only things that matter are the erase size and the pvt model but I am not 100% on that at all.
If you wanted Android 4.0 it is possible but they are all stripped and /or in BETA stage.
If you turn the phone off, turn it on again holding volume down you will come across a white screen.
What does it say?
I probably won't be able to tell you if you can root it or not but the more experienced people here should have more ideas about it with that information.