Blu-Ray (Blue ray, whatever) is really ONLY worthwhile with an HD tv (not just the receiver box). Because of (technical jargon), hooking a blue ray player to a standard def tv is like hooking up high speed internet to a commodore 64 computer (complete with audio cassette saving technology). I don't know how plasma tv's fare with HD; they really seemed to not catch on so much in the US.
As it stands now, I haven't heard of a BR player NOT playing DVD's. So, your collection is safe.
The prices are coming down too. For about $400 or so, you can get a smaller HDtv and a blue ray player.
Also, if anyone was paying attention in the HD-DVD vs. Blue Ray, Blue Ray won and all the HD-DVD's are going the way of BetaMax.
Everything is worth it in HD though.