If you look at the stats Valve picks up you'd see that an average mid range PC is still a single core 2.3Ghz to 2.6Ghz with 1Gb ram. And the graphics cards have taken a bit of a leap with the 7600 being about average now.
yes but thats based on all hardware out there. That a very different stat from what you would call a midrange PC that could be purchased today. Most PC gamers would consider the valve estimate to be low end in the current market.
I think both are valid definitions, it just depends on the context of your conversation. For example if someone said to you, "I have $1500 to spend, can you recommend me a good mid range gaming PC?" You wouldn't advise a 2.6Ghz single core with a 7600GT would you