By adding a $100 to $150 graphics card to (pretty much) any modern PC you will be able to play any game that is also available on the consoles at near maximum settings at 1080p at 60fps or higher ... If you start looking at the high end gaming PCs that have multiple graphics cards and a top of the line CPU they are probably more powerful that the hardware Sony or Microsoft will use in their next generation consoles.
The problem (as has been pointed out) comes down to games, and from a business perspective it doesn't make sense to target high end PCs anymore. A little more than a decade ago, people were upgrading their PCs every 12 to 24 months and games were much cheaper to develop, so producing a game that targeted the top 25% of systems upon release would soon be able to be played by most gamers; and you didn't need that many sales to break even or profit even if it only really sold well initially. To really push these systems to their limit, you would need to create a game that will sell in the 5 Million unit range (across platforms), and to do that without decent console ports at a time when people upgrade their PC every 4 or 5 years is not going to be possible.
With that said, I would expect to see more PC games that target higher end hardware in the near future. We're at a point where companies can complete games for the PC and (not too long after that) port them to next generation hardware at a time when game choices on those consoles will be limited.