[MrFoof]

I did this exercise about 2 years ago.

The price difference between a decked out mid-2011 iMac (with a DIY RAM upgrade) and a custom-built machine via NewEgg was ~$200 ... the difference largely being that with ordering from NewEgg I wouldn't pay sales tax. The difference with the DIY was I had a desktop video card instead of the mobile 6970M w/ 2GB of texture memory.

The other difference is I had separate speakers, webcam, more cabling, and a large physical box.

Although folks will fault the current iMac on storage expansion, don't rule out Thunderbolt. Yes, it's expensive, but being able to plug in a RAID array only loses points because the enclosure manufacturers always include disks to pad their margin. However, at least you can always plug that array into your next computer a few years down the line.