top showing up to 160% CPU usage
 

I'm trying to quantify the performance of a new s/w release and have been running "top -d1 -n600 -p920 > filename" to log CPU usage every second over a 10 minute period. A 3Ghz Pentium P4 gave a 10 minute average of ~68%. I then enabled hyper threading and get values peaking at 160 (% ??). Can this be regarded as 160 out of 200, or is it not as simplistic as that? If I average the hyper threading values I get 98, whereas dividing by 2 to get 49% would seem more appropriate

Hyperthreading involves taking a single processor and exposing it as two virtual processors, so yeah, it's pretty much 160/200 at that point.

Being a non-normalised number you can neither (safely) "average" the number nor "divide by 2".
You need to know the history of the usage - is one thread 100% busy on one "core", and the other 60% ?. 80/80 ? ...

But this is basically statistics I suppose, so use the numbers to prove whatever you want, just like everyone else does ... :p

In interactive mode you can toggle the CPU summary by hitting the '1' key to show individual cpus; see Section 3b http://linux.die.net/man/1/top