Try: Tigerdirect.com or newegg.com
I'm just giving you hints and pointers not to make a mistake. Take some leave some.. its your money. The 2 focusses you need to keep in mind is: Video card and RAM.
*For gaming you want an Nvidia video card, hands down. Not ATI because they aren't as good with updating the "shaders". It needs to say GDDR5 and at least 1GB.
*You want lightning fast RAM. It needs to say DDR3(at least) and at least 8 gigs.
Make sure you get x64 or 64 bit system to take advantage for more them 4 gigs of ram. (most systems nowdays are x64.
Hard drive thats at least 15000RPM for faster load time. The size moreless matters, its the speed you want to look into. 500GB is pretty much the lowest you want to go with nowdays.
Don't use the STOCK fan for your CPU. Upgrade the fan to a bigger one that fits your processor.
Powerfull machine = more heat
heat = bad for processor
+ if you get into computers enough, one day you might want to consider overclocking(boosting speed say 120%). For this you absolutly need bigger fan and heatsink.
700W power supply is the MINIMUM you want to have for a power supply.
Enjoy!