Actually, it's not. I've done it, currently am doing it. While it is a bit messy, it works GREAT!
On a REALISTIC note, a 800D will get very toasty. you may need to replace the stock fans with some AP15s, or get that dremel out.
IF you have your eyes set out on the 800D
- Place a 120MM fan in the front 5.25 bays
- 3 120mms in the back
- new fan to replace the middle power supply area fan.
- Maybe dremel out the side panel for a 200MM if your feeling lucky