First you need to decide on $ budget for your comp
Second what type of games, i.e. If you want to play Doom3 Quake4 Call of Duty 2 you are going to need a higher end comp.

Once you get a budget going you can start looking at mobo/CPU/Vidcard choices.
Vid cards I like ATI so for a lower end comp x1300 is a good choice $130 to $150
High end then you will want x1900 crossfire pair (two cards) $1100

I am a AMD fan so I would choose a X2 dual core check for prices on your budget

MoBo now that you know the vidcard and CPU you can choose a MoBo made for them

I like ASUS best

Then you Style, i.e. case lights, does it need to be mobile?

Also buy your parts online or at a comp show, much cheaper than stores