What is the difference between building the machine I posted and buying this one from Dell?
Customizable XPS 8500
Dell Wireless 1703 802.11b/g/n, Bluetooth v4.0+LE
Integrated 7.1 with WAVE MAXXAudio 4
Dell Laser Mouse
Dell KB213 Wired Multimedia Keyboard, US-English
16X CD/DVD burner
AMD Radeon™ HD 7770 2GB GDDR5
2TB 7200 RPM SATA Hard Drive 6.0 Gb/s
Memory 12GB Dual Channel DD.
Windows 8, 64-bit, English
3rd Generation Intel® Core™ i7-3770 processor (up to 3.90 GHz)
“All that is necessary for the triumph of evil is for good men to do nothing." ~ Edmund Burke