The trouble with buying a pre-built vs building your own, is that you likely won't be able to overclock the pre-build. If the board isn't locked out, than you still will likely need to replace the power supply, and the chip cooler.

 

I saw that... Really, there's no need to flinch when you here “Overclock” these days. Modern motherboards make it very easy for a novice, and the core i7 begs to get cranked up a little. I'm just running mine on air cooling, and it was very easy to get it to 3.8GHz. At 4GHz, it starts to get a little too hot. However, Corsair makes the Hydro H50- which is a fully self contained water cooler that fit's the i7 perfectly for about $70. From what I've read, the i7 920 will run happily at 4.2GHz with little effort. That will be my next upgrade.

 

Besides, when you build your own, you get to decide exactly what you want in the system, and what you don't really need- both hardware, and software.