It sounds like you have a decent reference point. I would check some CPU benchmark scores of your current desktop computer and put those against any laptop CPU you're interested in. You can't directly compare GHz and numbering between mobile and desktop CPUs but benchmarks will give a rough indication of the performance difference. 8GB is a decent amount of RAM, I personally wouldn't go lower than that.

Final comment:
 
I would only consider downgrading if it was necessary, but I don't think it will be.
 
As an aside, if anyone can help resolve the issues on the desktop, they generally only happen when I am driving things really hard with some heavy duty processing. I have just loaded a project with four instances of Z3ta+2 and two of Reaktor 5 (Lazer Bass and Akkord instruments)and 3 audio tracks with no real problems (cpu monitoring showed <20% utilisation but peaking at 42%): but I do occasionally get dropouts and glitches - mainly with Reaktor.
 
My audio interface is an old M Audio Fasttrack.
 
Anyway, thanks for all the help, and as I say any thought about tuning the sound card drivers would be appreciated. I do have the latest and they are certified for Win 7.
 
Regards
 
roger