Depends on what you mean.
Are you asking what the specs for an Uber awesome rig that will probably play any game released in the next 5 years at max specs, or just what the average workhorse mid-ranged PC that will run most games at decent settings specs are?
An Uber rig will have a 4-6 core CPU probably running at around 3.1+ghz, two or three high end GPUs in SLI or Crossfire, 16gb of Ram, a high end MOBO, at least 1 1TB+ HDD for data storage and another 500gb or so SSD for the OS and graphically intensive games, a 1200-1500w PSU, after market cooling, a large custom case, and probably a Blu-Ray drive, with possibly a DVD-RW drive as well. I've got something like this and it's stupidly powerful. There are no games out at the moment that can take advantage of this much power. It's overkill and future proofing to do something like this. When I saw the specs for the PS4 and Xbox 1 I chuckled a little bit at how cute they both were.
That's very expensive, but it will last you a really long time if you take care of it.
On the other hand, a workhorse rig that will run most games well is more like a quad core CPU at around 2.7-3.0ghz, the best single GPU you can afford, 8gb of Ram, an 800w PSU, a 1TB HDD [if it's primarily for games I recommend at least that much, you'd be surprised how fast it fills up], and a DVD or DVD-RW drive. You can easily build something with this model that will outperform the PS4 and Xbox 1. Though, you would need a high end GPU for that.
Either way, as long as you make sure you get a good Mobo you can upgrade easily for a lot less than another console will cost you. Also, you'll be wanting a 64 bit OS. 32bit isn't horrible and works fine for a casual PC, but most newer games take advantage of a 64bit OS so you really do want one. So, for a gaming rig Windows 7 64bit is the best choice. Windows 8 is kind of crap and a lot of games don't like it. It was primarily designed for mobile devices and tablets and got a PC port for no good reason. Avoid it and stick with Win 7 64, it's the best OS available for a gaming machine right now.
If you want to primarily play games on something I don't recommend just going out and buying something 'new' and hoping it's enough. A lot of Laptops and Desktop PCs that are prebuilt aren't made with games in mind, but rather browsing and business. You're probably not going to find a great gaming rig on the shelf at Walmart. Even Bestbuy isn't going to have much catered to gaming and tends to stock more towards casual use.
The cheapest way to do it is build your own, but if you're not into that I recommend having someone build you one. Off the shelf PCs aren't very good gaming machines especially if you're looking to play games like the newest COD, Battlefield 3 or 4, or Farcry 3.
It's going to cost more than a console initially for sure, but despite the higher up front cost it actually costs less in the long run. Games are cheaper, you don't need to pay for multiplayer, and there's a lot more free stuff in PC gaming's world.
