Well, I agree with what the guy above said: custom is cheaper. And, you learn all about computers and hard ware and what makes them work, which makes them a lot easier to troubleshoot along the way. And, when you go custom, you don't have to get everything at once. What I mean by that is that you could get only 512 megabytes of RAM now, and then upgrade to 1.5 gigabytes of RAM later on. I don't mean it like you don't have to get a video card now; haha, you'll need one of those to run your machine!
And customizing computers is sometimes the best way to get what you want, but usually you won't get as good of a deal as you would if you bought a pre-made computer because of rebates and such.
Bottom line: build your computer! You won't regret it!