Well...I guess that my first question would be...what do you consider to be a "media center PC"?
If it's a PC that uses XP Media Center as the O/S...bear in mind that the same effects can be achieved without any version of MS MCE.
Many of us have had ATI or other tuner cards (which give us the capability of watching/recording/editing TV) for several years now.
These have been available long enough for ATI to discontinue its All-In-Wonder line of video cards and be bought out by AMD...and then reinstitute a new series of A-I-W products.
From what I see...a "media center PC" is something that manufacturers and Microsoft dreamed up to take advantage of the fact that many new PC buyers...had no clue that any PC could easily be transformed into a multimedia center.
FWIW: This is what Wiki says, http://en.wikipedia.org/wiki/Media_center
Marketing, marketing, marketing...IMO.
What is it that you want your "media center PC" to do?
My last question...is where did you get these component minimums from? About the only things you listed that I can categorically agree with are the RAM and the hard drive.
Rendering video on a computer benefits from as much RAM as possible, while watching video requires a negligible amount of video RAM (certainly not 512MB). Rendering video has nothing to do with video RAM.
A large hard drive is a good thing, due to the size of video files (when saved). If you don't intend to save them, there's no reason for a drive larger than 120GB or so, IMO. (Today, there is little point in buying a hard drive that small, 1TB drives are available for approximately $100).
Best thing I can suggest...plenty of reading before deciding what you want. In computerdom, there are several ways to achieve the same ends...often.
I may not be understanding the capabilities of what is marketed today as a "media center PC" and I apologize if there really is something other than marketing behind that tag.