It's not terribly expensive to build a gaming machine. But if you don't need a new machine generally I don't recommend it. But you can do one for under $ 800 easily these days. Whenever I build a new computer I always make sure its gaming compatible.
http://www.newegg.com/Product/Product.a ... 6822148767" onclick="window.open(this.href);return false;
Half a Terrabyte HDD is $ 40
http://www.newegg.com/Product/Product.a ... 6819115211" onclick="window.open(this.href);return false;
i7 3.06ghz Quad core is $ 360 (This could be a place to save a hundred bucks by going with prior gen gear)
http://www.newegg.com/Product/Product.a ... 6813131665" onclick="window.open(this.href);return false;
Motherboard $ 200 (This one isn't super great with a max of 6 Gigs of RAM capacity. But 6 gigs is fine for any game.)
http://www.newegg.com/Product/Product.a ... 6814500193" onclick="window.open(this.href);return false;
Graphics Card $ 135 (This is the most important piece, don't go any lower than this. Can go much higher. The Graphics card is the place where you have the most variability on performance. 1GB of RAM minimum for gaming.)
http://www.newegg.com/Product/Product.a ... 6827135204" onclick="window.open(this.href);return false;
DVD Burner $ 20
http://www.newegg.com/Product/Product.a ... 68201482423 GB RAM $ 60.
http://www.newegg.com/Product/Product.a ... 6811133183" onclick="window.open(this.href);return false;
Gaming Case $ 85 (Very important that it keeps it cool as modern processors run very hot, so your CPU and GPU will create a lot of heat)
Oh wait, I forgot the Windows OEM.
http://www.newegg.com/Product/Product.a ... 6832116986$ 100.
So this machine would cost you $ 1000. It's just what I eyeballed. You can get a 22 in flatscreen for about $ 100 now. You can spend as much as you want on a gaming mouse and it indeed DOES make a difference in gameplay. Gaming mice are between $ 50 and $ 250.
Gaming is no longer the most intense thing one can do on a computer. Audio/Graphics/Video production are really what you need high performance for. The machine I pointed out here should run every game on the market at the second highest graphics settings if not the highest.
The CPU is where you can save money on this machine really. If anyone is in the market, I do not recommend buying this machine that I have outlined because it was just a quick and dirty mock-up and I wouldn't want people to buy incompatible parts. But the compatible parts will be comparably priced. It's all about ports. Your Graphics card should be PCI-Express. RAM these days is all DDR, make sure the motherboard supports the particular type of RAM you are getting. The Motherboard and CPU are a mated pair so you have to get the right Motherboard for the CPU that you buy.
The canary didn't die because this mine is dangerous, it died because it's lazy and wasn't raised with a proper work ethic.