MH--
I think most of the posters on that thread were not saying "this is the minimum", but rather saying "this is what worked for me". Also, many posters mentioned what kind of screen resolution worked flawlessly, and at what res the playback might have gotten jumpy, if it did at all. I think if you read through that you'll get an idea of what's going to work fine. While RO HQ is fairly taxing on resources, RO Standard is much lighter, and darn good looking also.
I wish I could help you more with this, other than to say it's going to require a decent proc (my i7m worked fine), along with a reasonably good 2d video card (my 2gig ati worked fine). I would note that if you're looking to get away as cheap as you can, it is PROBABLY wise to stick with an Intel CPU, as the consumer AMD's seem to be missing odd bits, and at slower speeds that may matter.
Also, make sure your anti-virus is set to allow Media Center and JR worker to do their stuff.
Also worth noting, you get a free trial of MC, so maybe try it on the machine you have now and see how it flies. Good luck!