Did I fail to mention it's addictive to build your own computer? Well, it is! When I put together my first PC I spent way more than I should have because I bought things simply for the "WOW" factor, whether I would get any use out of them or not.
When you first start building a computer set yourself a budget, somewhere between $750 to $1500 is a good range. Any less and you'll be selling yourself short, but anymore and you could have bought a pre-built PC for less money.
In addition, you can go above this and set yourself a monthly or annual computer budget. Personally, I limit myself to one mod a month so that I don't go on a shopping spree spending more than I should.
Also, don't be taken by the claims that you "need" the latest "wiz-bang" computer part. Whenever a product is first developed, the price is generally outrageous and the product has a ton of bugs. Generally if you wait a half-year, the products kinks will be ironed out and the price will have drastically dropped.