Since I don't live in the u.s. I'm not too fond with the shops there but I heard newegg has decent hardware. Well whatever shop you pick these are important requirements:
-at least quadcore with 2.5Ghz per core
-64bit Windows 7 (Home Premium or better)
-4GB DDR3 RAM
-a fast dedicated GPU (GTX460, GTX560, HD5870, HD6870 or better for high settings in all games, the second number in the name is important and shouldn't be less than 5 for nv and 7 for AMD, so GTX550 is fine but 540 is not)
-PSU with at least 400W (and maybe a brand people know like Corsair or OCZ)
-at least the size of a midi tower so you can replace/add parts
-you should also google for tests and customer reviews before buying to see if theres problems with the product (like bad quality of loud noise)
-the HDD is not that important since it's an easily upgradable part (and pretty cheap)
-stuff like 7.1 surround, hdmi, Gigabit lan is in almost every mainboard so don't get blinded by big texts announcing "this amazing feature"

If I forgot something feel free to add xD