Actually, I disagree with the general view that "building is cheaper!"
OEM companies apply a huge discount to all the hardware they package together. On the hardware ALONE it can be cheaper, and that's not even factoring in the fact that it usually comes with a Windows license and other software, plus being pre-assembled.
For instance, I bought my Dell quadcore desktop, 2GB RAM, and two 320GB hard drives for $400 - pretty killer deal. At the time I bought it, a new intel quadcore processor ALONE would have run me about $400... not to mention the case, power supply, and motherboard. I started with this setup, then upgraded parts individually... transplanted into a new case, then later swapped all the parts to a new motherboard, then a new power supply. Honestly, I'd say that $400 was worth it just for the memory, processor, and hard drives.
Usually when we're talking about the average person, pre-built PCs are actually cheaper these days. It's been this way for a few years believe it or not (I used to manage a computer store so I'm well aware of the prices) ... now when you're buying premium stuff like Alienware where you end up paying for brand, yes it might be cheaper to build, but it also might not be the best option anyway.