The Extron 202 Plus is HD compatible and has some additional adjustments that the lesser boxes don't. I would recommend a Mike Parker modified Radeon card (MP-1) over the Extron, if you are thinking about using it with an HTPC. I've tried both.
If you just want to use it with an HDTV box, I don't think it's necessary unless you have a very long run to your projector.
Mark.