>>In contrast, on the Pentium D 915, your app would get at most 2.80GHz because it can't take advantage of more than 1 core. Of course, you could run 2 copies of your app ;)
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>>This scenario is a bit artificial; in practice, you'd probably upgrade to a Core 2 Duo, which gets large gains even at lower clock speeds because it's more efficient - does more work per clock cycle.
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>Haven't tried this yet - still haven't installed any of my apps on the laptop, which has this double-headed dragon inside - so this is also a mental experiment: if I had a queued part of the application, which would perform tedious tasks for the primary part, and this queue app would be a separate exe, then they'd be each playing in their own sandboxes, wouldn't they?

Depends how you call the tedious/queued part. If you call it via a DO or function call in Primary, it would still be running in the same thread (i.e. synchronous). It would need to be started separately through an OS/API call (i.e. asynchronous).

Laptops tend to have slow hard drives so if you speed test you might need to make sure you've got lots of free RAM and your data get cached.

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>Is there some special switch when running ShellExecute() to tell the OS to fire it in the other core (if available), or should I rely on the OS being smart enough to do so automagically?

A lot of work goes into modern OS schedulers which load balance automagically to the best of their ability - and which generally do a pretty good job. I think processor affinity can be specified, but I don't know how, or if it can be done through ShellExecute() - I think Thomas has some knowledge in this area.