I totally agree with Firefox, building a PC yourself is really worth it. First of all you can learn from your mistakes (for example not to fry a CPU, and trust me I not laughing at that, I've done a lot of things like that
I had a dull... I mean dell, and it's just not perfect no matter how much you customize it from pc manufacturers, and plus they don't even give you the parts that you want. Not having a PC you like is not really cool, and like ChatMod#1 said you won't have the satisfaction either.
So just go and get yourself all the parts (no matter how many times you need to get them) and build it yourself, that's what I do atleast. Although it might not be as cheap as dell's or it might not be as easy as a barbone kit, it's still worth it!
The only thing that's sort of a side effect is friends and family coming to you and beging you to make help them build a computer (I'm sure William has experienced this as well)...