Another thing to watch out for is the motherboard on mass-produced machines. These are usually of relatively poor quality with sound/graphics and even modem built-in. This can appeal to some, but if you are serious about PCs then you will want to customize your PC with the latest graphics card, etc if you are a gamer. Imagine your dismay when you open up your PC to find that you have an integrated AGP card, which means that (a) you can't take the old one out, and (b) because the AGP is integrated, you'd have to get a refund for your new AGP card and get an old PCI one instead. Similar scenario if the graphics/sound develops a fault and you want to repair it.
You can usually get a good deal at computer fairs, but be careful at these events. They have, in the past, had quite a bad reputation for being a forum to sell-on faulty equipment. Nowadays, many of the stall holders own small businesses or shops who will be happy to provide you with a contact number should the device be faulty when you come to plug it in. Try to go with someone who knows the PC market well, if you don't.
The deals are out there, but they're just a little harder to find than before.