I absolutely endorse NewEgg for their shipping speeds, which seem to have harnessed wormhole technology.
As for hardware, here is my recommendations for what you need for a good minimum.
560 series or higher Nvidia card or equivilant. This is widely considered the "entry level" for video cards in gaming rigs.
Quad-core that is at least 2.5Ghz.
A motherboard that is compatible with all of your parts. Get one bundled with the CPU for ensured compatibility and a discount.
8GB of DDR3 RAM; 4 sticks of 2GB is preferable but 2 sticks of 4GB will work.
A monitor that supports 1920*1080.
A 750W power source.
If you want to spend a little more and want good load times, get a small SSD and put often-used games and software on there while dumping everything else on your HDD.
Above everything else, check the reviews for products and make sure there are multiple and consistent good reviews.
If you can, see if you can get everything in a DIY Kit Combo. It will include the parts and just require assembly, you'll save money overall, and there is no tacked on fee like that of pre-built PCs.