If you're getting anything over the counter, it'll usually be second rate. If you don't have the knowledge to put a computer together yourself, here's a few pointers to remember:

Firstly, Windows 7 has memory limits; 2 GB for anything under Premium, 4 for anything under Ultimate, and Ultimate has 8GB. 64bit handles memory better and is overall more efficient, but I won't bore you with why.

Secondly, don't be sold on hard drive size. it takes about two minutes to install a second HDD, and seriously; if you're using anything over a Terrabyte, you're just being silly.

Third, if they advertise anything over a dual core, ask about the cooling system, then go to a computer hardware store and ask them if the CPU cooling system will do the job. Often quad core computers and inefficient cooling, and will rare have more than 60% of the resources usable.

Lastly, ask about the expansion slots on the motherboard and the power supply. If you look online, you should be able to find any graphic card's power demand easy enough. Add about 400 watts for system general and headroom, any extra is power that can be used for upgrades later.

Hope that helps!