Here is a good example of a (4 years old) almost completely fanless pc. well there is one big fan - super low RPM (= no noise) on the power supply positioned inside, sucking the air directly from the specially designed cpu heatsink.
Video: Geforce 8600 GT fanless x2

With recent version of K-litemega (on overlay mixer) CPU is never more than 10% playing 1080p/i.
It is powerful enough to play around 8 HD streams (720P) at the same time.
Recently removed the nosiest part of the PC - 7200 rmp hd - replaced them with SDHD and 5400rmp combo.
You have to press your ear very tight against the case in completely silent room to hear anything...
P.S. I am very surprised you need such powerful video (GT220??)that you mentioned in you post for HD stream...