This of course only applies to the older PIII / P4 Xeons - not the latest Woodcrest based chips. But then again, the older PIII / P4 Xeons are the most common targets of P2V migrations, so it works nicely.
The formula is:
(AMD MHz + 500) X 1.5 = Intel MHz
I know you want to confirm it for yourself, so go ahead - here's a list of a few Athlon64 CPU's with their PR & respective MHz numbers.
So, if you had a dual 1.4GHz PIII Xeon box using 20% CPU on average, and you wanted to know what that would equate to on a single 2.2GHz Opteron, you'd do something like:
Dual 1.4GHz PIII Average Workload in MHz:
(1400 X 2) X 20% = 560
Estimated Total Intel Equivalent MHz of Single 2.2GHz Opteron:
(2200 + 500) X 1.5 = 4050
Estimated Percentage CPU Utilisation of Single 2.2GHz Opteron:
560 / 4050 = 13.8%
Of course there is a boatload of confounding variables, such as how optimised the application is for multi procs, is the Opteron dual core, what if that was a Hyperthreaded P4 Xeon instead of a PIII, and all the architectural differences in the chipsets etc etc. While they are all valid questions, I'm just talking about a very basic rule of thumb here. You've gotta draw a line in the sand somewhere, and there's little point in trying to make a guestimate more accurate - in the end, it's still a guestimate.
If anyone else out there knows of a better / more accurate conversion, please post it in the comments!