Any PC gamer will tell you that buying a pre-built computer is an extremely bad idea, because they are overpriced. For about $1,000, you can built an awesome computer that will destroy almost every game out there.
Advantages?
1. INI tweaking(very useful in some cases)
2. Higher resolutions(most console games only run at 1280x720, and so you can't full utilize that 1080P HDTV, unlike with a PC)
3. Better performance(consoles are locked to 30 FPS)
4. Keyboard and mouse. This is my personal opinion, but once you get used to the keyboard and mouse, there is no going back to controllers. Being able to make a 180 degree turn, kill an enemy, and make another 180 degree turn in about a second is a luxury I love.
5. Better graphics. This is a given. PC games have options and sliders and everything when it comes to graphics. Completely adjustable, with INI tweaking, you can make things such as view distances, even higher.
As for my system, it was a pretty mediocre HP my dad bought a couple of years ago for about $700(overpayed). We then upgraded the RAM, PSU, and GPU, and although any PC gamer will tell you it is an "okayish" computer, it still runs most games well on High settings.