Go with an Nvidia card if you can. Anything with a GTX on it is class. While Radeon is an attractive bang for buck proposition, I've had bad experiences with the drivers to the point that some games won't work properly at all. This may have changed, but Nvidia is always ahead on drivers anyway so its a safer bet. Personally, I run with two 670GTX's on SLI(two cards working at the same time) and it runs everything fine. 700GTX series are even more powerful and efficient, so one card would do the job easily. There's pretty affordable ones right now on sites like newegg.com and even at some retail stores. You don't need the top of the line, but its always good to future proof if you can afford it. Otherwise, the GT series is still efficient but you might run into slowdown at the higher resolutions. More power on your videocard means that you can get rid of the dreaded jagged look that plagues most consoles games (although this has gotten better with this new generation).
As far as processors, you'll have to see what you want to spend. I wouldn't really go for any of the "Extreme" series from Intel because they are way too expensive and seem like overkill, but I would definitely get an i7 with at least 3+GHz. Some people prefer going higher, but it usually involves overclocking and that will reduce the lifetime of your processor.
In terms of RAM, get at least 8GB. That will give you enough room to run your OS as well as any other applications and games without any hassle. If you're going to be doing 3D modeling and/or video editing, I suggest to go higher, but otherwise, it's overkill. Most games don't even come close to using 4GB's of memory so with 8Gigs you are safe.
For a power supply, I suggest something with plenty of power (+800watts) but no need to go reeeeeaaaally high. The trick with these is to find some that operate quietly because with a high end video card and a loud power supply you will end up with a PC that sounds like a jet engine like mine, specially if your case is well ventilated.
Other nice addons are a water pump for the CPU so it keeps it nice, efficient and cool. A well ventilated case works great too. Even though I'm running a lot of power, my PC doesn't even get nearly as hot as the newer consoles because of the liquid cooling.
If you select the parts yourself, you sill save quite a lot than buying pre-built. Brand and designer gaming PC's lure you in with the lower spec price but if you actually choose parts that can run games on max you end up paying a ton more than what was initially advertised. Some are beautiful, but they run into the same problems as self built machines. I've had an Alienware desktop before with all the bells and whistles and while it looked beautiful and ran quiet, it was no more powerful than what I had now and had several problems with the BIOS out of the box as well as costing me almost twice of what I paid for my self built. And that includes paying a store to build it for me.
You won't regret it if you're into beautiful visuals, it is well worth it in that regard. There's nothing more beautiful than NBA2k at Max settings running at a smooth framerate + all the mods.