Edge and HC both give good references for brainstorming your build. So, there is nothing really I need to say in that department. (This is what I like...I do enjoy when you guys ask me questions, but in the end, I also want you to learn about computers and make those kind of build decisions for yourself...it makes it that much more fun, and makes building less intimidating.)
As they both have already stated, anyone can decently game building their own rig without breaking the bank. Keep in mind, it can be cheaper to have a pre-built mainstream rig than to build up one yourself.
You build one because you want to...and you know exactly which parts you want. (After all, the talk is that the age of desktop PC gaming is dying!)
As far as what to spend? Granted that chart shows that there are rigs that can be had under 500.00, but seriously, anything under that price tag isn't going to have much longevity unless you know for sure you are going to be a casual gamer.
There are a lot of references you can use. Tom's Hardware has a system builder's marathon with different class price deltas and benchmarks to give you an idea of what you might want to run or if you want more.
500.00 Value PC - June 2011 1k Enthusiast PC w/ 2x 460GTX in SLI, SSD hard drive, and overclocked 2500K processor - Sept. 2011.2k "Performance" Build - Sept 2011.The verdict is out on the 500.00 build for Sept 2011, keep checking in with them, and they will eventually have the report and then do a comparison of all three rigs for the month.
For example: You could easily take off probably another 200.00 or more off the enthusiast build because you really wouldn't need 2 460GTX's SLI'ed, 1 460GTX would be enough, nor is an SSD boot drive necessary...and you can build from there.
Tom's HW also has a monthly CPU and GPU "best bang for the buck" article that states the best processor and graphics card by price deltas, and then the conclusion with an a comparative hierarchy chart to show which camp compares to whom.
A lot of what creates what your build will become is how much you can realistically afford without having 3 squares of ramen a day for half a year.