If you're not going to be building and you really have no budget then there are some minimums for a decent gaming machine using only 1 screen:
CPU: Intel 2500k or greater OR Intel 3820 or greater
GPU: ATI Radeon 7850 or greater OR nVidia GeForce GTX 560Ti or greater OR nVidia GeForce GTX 680
RAM: 4 GB minimum
OS: Win 7 64-bit
HD: Depends on how painful load times are for you. Once you're in game you won't notice any difference with a faster hard drive unless you're changing stages
Use one of the digital methods of connecting your video card to your monitor (DVI/HDMI/Displayport) and run at the native resolution which is usually the max res your monitor can support
If you're going to buy this at a mom & pop shop then make sure you're getting a decent PSU and motherboard also - this will aid with system stability. If you're getting it from one of the big name guys like Dell (Alienware) or Falcon Northwest then you won't have too many choices.
If you have any plans to overclock your system make sure everything is nice and stable at standard clocks first. Once you've broken it in for a couple of days and it's rock solid then you can start trying to break it.