Is this even the right forum to ask?

How do you start figuring the processor capacity needed for an SQL
Sever when there isn't much relevant experience with the application?
Some people's gut feeling is that two active nodes with quad
processors will be adequate, others think it will require
eight-processor nodes.

Would you try to approximate the load with tpc benchmarks and then
compare to published benchmarks?

There is some performance data using simulated users on a single
quad-processor box, but the simulated users are much busier than real
users in order to generate the load - would you try to scale this up
by number of processors and real world activity factors? This
approach is further complicated because the activity to create this
data identified some opportunities to improve the performance, so
scaling for anticipated inprovements would be needed, too.

Any other ideas about where to start? Any web sites or consulting
services that are likely to help?

William