is worth it to get a high end PC for playing games at high framerates/better performance et.c
That part

It used to be: good computer = good performance.
But I'm getting the idea that this is no longer the case.
It seems to me it has changed to this: good computer = greater chance of running it well, but no guarantees.
A little background, as to where this question came from:
I was thinking about buying a new gaming computer. My current machine is 5 years old, and was a medium range gaming machine. So now it is a low-end gaming pc.
One day, I was over at a friend's. He had Skyrim for the pc. His computer, comparable to mine, ran Skyrim pretty well on low settings. Constant just below 30 fps, dropping to just above 20 in cities, and maybe just below 20 when there is a lot of fog.
Anyway, his brother has an almost-1-year-old, high end gaming pc. It runs almost any game better then my friend's pc does. But Skyrim is almost unplayable on this machine. No matter the settings, noticeable and annoying lag is always present.
And I have more experiences like this. My pc runs Borderlands pretty well on medium-low. While another friend of mine, again with a better pc, can't play this game due to graphic glitches and lag.
So, I am a bit worried about spending a lot of money of on a gaming computer, when it seems uncertain if this will guarantee better/good performance with games.