My friends are now all PC gamers and I would love to be able to play with them, but my laptop cannot handle anything besides the most graphically basic of games (it cannot even run Left 4 Dead very well, which isn't the most taxing of games thanks to the Source engine). Whilst I too would like to enter PC gaming, as I am frankly disappointed with all of the currently announced/available next-generation consoles and love the community surrounding PC gaming, I simply don't have the money to buy the sort of rig I would ideally like. Consequently, I want to know what is the cheapest gaming PC I can buy/make, which I could then upgrade to get better graphics (I just want to run the games at a solid FPS and console-equivalent graphics to begin with)?

Honestly, it depends what you have. If you already have a keyboard, mouse, and monitor, that could save you a a bit. Otherwise, you'd need no less than $800.

I am pretty certain that I have a mouse and keyboard somewhere, but not gaming ones. Though I suppose I could use a regular mouse/keyboard and then upgrade them later. I may be able to use a Samsung monitor off another PC, as it is potentially getting hooked-up to a television.

It may be slightly dated now and prices may vary depending on where you buy the hardware from, but the 'Digital Foundry PC' seems nice. http://www.eurogamer.net/articles/df-hardware-introducing-the-digital-foundry-pc