Perform an uncompressed, raw rip of the CD to 44.1 kHz 16 bit wav and play that on your PC, versus a compressed rip. I just compared a .flac track with its .mp3 counterpart and any differences are negligable. Which is kinda besides the point - Joker is saying in tests plenty of folk can't tell the difference, whether we can or not, so the difference between CD quality and production quality doesn't make sense.
As mentioned, most PCs can be pretty horrible for this kind of test. The iPhone and PSP output way better quality sound over the headphones than any analog output of my PC. Good mp3 files can definitely be good enough for most music, but most music also isn't designed to make the most of high frequency sounds (always have to think of the rap lyric "Burn the Bass, Turn up the Treble!" in The Power, which actually does use a high frequency sound, but it's not in the range I'm talking about now).
Incidentally, how much processing power is enough for audio also depends on your ambitions. Some games do a little bit more than just pop up sounds behind you. Uncharted 2 is a good example, not just of high quality audio, but also of taking into account occluding objects, which can give pretty spectacular results. These I would suggest fall into the category where people just think it sounds great without necessarily noticing why.
Also, yes, I always make sure I'm sitting in the right location. But even when I don't sit in the right location, if the sound design is really good, the location of the sound versus the screen is still correct. But starting with a humble Pro Logic set back in 1993, just the simulation of being in the room that the movie is in alone was such a huge win. I remember that movies like Speed, if you were in an elevator, going into the elevator shaft, in the bus, in a home, sounds reverb accordingly (your rear speakers can be setup to take into account how far they are behind you for the optimal effect - that system had small, medium or large distance settings, but that already mattered a surprising amount).
That's not to say that it matters always. There are a lot of games and movies that use crappy source material, and even share them (Doom has the same sliding door sound as many a space movie). And not everyone cares. My wife has taken a while to get used to a good setup. She found the surround highly unsettling at times because she sometimes can't distinguish if something is a movie background sound or if there is actually a baby crying, phone ringing, or whatever.
I've also had fun with our cat hearing kittens and calling back to them, trying to find where they were around the tv.
Anyway, let me pick the source material and I guarantee you even most untrained ears will hear the difference for high frequency sounds (though keep out the djs, dance junkies, large concert goers and certain brands of audiophiles, as they all probably have way too damaged hearing anyway).
EDIT: yes, a triangle is definitely one of the best single source high frequency sounds you can test with.