Things to look for...
If you are going for the newest generation of processors, make sure to go AMD. The newest pentiums run so hot you'll probably wind up with a very loud cooling solution.
When it comes to graphics cards, there is no practical difference between AGP and PCI express currently so don't be fooled. I'd recomend going for a radian X700 or a GeForce 6600, they will give you all the bang of their bigger brothers at half the cost.
As for RAM, 512mb is the bare minimum going up to 1Gb. However, don't sacrifice quality and speed for size, you'll only regret it later.
When working on a budget, stay the hell away from anything flat. Even the best ones don't have the same kind of quality and versatility of your everyday CRT. Especially since you are looking for a gaming rig, it would be a big mistake.
As for the motherboard, look for something name brand. This is usually something that most people look over in getting a new PC. If the brand is ASUS, MSI, AOPEN or Gigabyte, all the better.
Most of all don't be afraid to run a google search on something you arn't sure about or don't quite understand. If that doesn't work, websites like www.newegg.com and www.tomshardware.com can really help you understand what the different names and specs actually mean.