I used to be a right computer gaming geek overcolocking them and all that lark.
Basically any newish I5 processor, 8gb of decent RAM and a 7200rpm hard drive and 600-800w PSU is all you need. Then spend the money on a decent graphics card, preferably one with an aftermarket/uprated cooler. The newer chips are made on a smaller process so will generate a lot less heat for a given power/performance. I would recommend a Nvidia 570,580,660,670 or ATI HD7870,7850,7950. Check here for a breakdown of best cards for the money at different price levels.
After that, just get a well-ventilated case (bigger fans are generally quieter for the amount of cooling.
EDIT : Oh and a word on monitors, the graphics card you need will depend on the resolution you’re playing at. Below 1080p you should concentrate on the chip’s speed wheras at 1080p and above the memory becomes a bit more important (e.g. 2gb and ddr5 memory).