Ever get the feeling people treat this place with a need to instantly express whatever deluded opinion pops into their head? Most of the world would suffice with a Pentium 4, it's called marketing and you've bought a front row seat.
To the OP, I'd personally suggest buying the cheapest Mac you can, besides a few (or several) minutes here or there on handbrake encoding times any Mac sold in the last 4 or 5 years can handle the tasks you describe. Obviously you want the iMac for the 27" screen, which is fine but go for the cheapest option.
I do everything you just described and more on the baseline MacBook Air with 4GB RAM, with half my RAM being used by Server 2008 in a virtual machine. Absolutely no slow downs, not one, nada, zilch.