Out of 2 possible keyword combinations, like HouseDog/DogHouse, UsedCars/CarsUsed, DomainForum/ForumDomain, the one with more Google search results is usually the better, because of reasons Reece already mentioned - it is usually the more naturally occuring sequence.
If you are unsure of which word combo is better, Google seacrch results can help decide.
However, note that this is only good for relative superiority, and not always even that. Like Reece said, you can't use Search REsults alone to determine the value of a domain, because Google does not account for punctuation between words.
Case in point: The domain WikipediaThe.com -> the keywords "wikipedia the" in quotation marks result in 28 million google results. Still not a good domain.
Domain value comes from various properties: Among these, search popularity and phrase frequency are great for a quick overview, but these are of course not sufficient, nor are they essential. You've got brandability, PPC value, memorability, descriptiveness, market size, age, and dozens more.
I agree with Reece: among the best indicators of a good domain is whether it sounds good or not.
My personal, #1 indicator is: What could this domain be used for? Who would benefit from having their website developed on this particular domain?
Ask yourself: Who needs this domain? Why do they need it? Why is it essential for them to have it? How can you justify paying hundreds if not thousands of dollars to have this particular domain? What's the advantage, the benefit, the market edge?
If you can't answer those, the domain is not worth getting.
Whoever here once said that you have to count the number of things to remember in a domain, I though that was brilliant. I forget where the original post is, so I'll try to summarize it here:
Take UsedCars.com
-> zero things to remember. Describes the product exactly, and the .com TLD is the default. The absolute best domain for selling used cars.
UsedCars.net
-> One thing to remember: the .net extension (will lose traffic to .com)
UsedCarz.com
-> One thing to remember: The letter 'Z'. Still a pretty good domain.
UsedCarz.net
-> Two things to remember: The Z and .net
Used-Cars.com
-> One thing to remember: the hyphen
Zaboom.net
-> Three things to remember: The domain itself, Zaboom, and the fact that it was a used cars website (that's why UsedCarz.net is better) and the .net TLD