Hi edeng,
I have a media PC in the lounge rigged up to my amp and a 42" plasma as the display. I use "videolan"
http://www.videolan.org as the player it's free, but I also use Power dvd 6 it just struggles more than video lan with streaming from the network.
I'm cheating really though
I force the High Def through the VGA input of my plasma screen at 1024 x 768, I was too eager with the plasma and bought a Panasonic P7
which isn't compatible with HDMI or DVI inputs that fully support the High Def standards of 720i or 1080i, but even with that in mind the high def stuff looks really good on my plasma once you set the interlacing up correctly
IMHO it's a better picture than standard dvd. A friend of mine has a Hitachi fully High Def compatible plasma and the HD stuff connected properly via DVI looks absolutley superb on that, it's like looking through a window not at a screen, if you think back to the first cd you listened to compared to a vinyl disk that is what true high def is like compared to dvd it really is that good.
When HD tv via SKY kicks off in the UK next year the plan is to upgrade my plasma to the P8HD Panasonic which is basically the same screen but is fully HD compatible, and change the amp to one that can upscale and switch the video input to a HDMI or DVI signal
To get an idea try downloading some of Windows HD clips, they're reallly good and give you a great demonstration of what HD looks like, anyone with Windows MP10 and a monitor that can display 1024 x 768 can view true HD images