Brown,
Lets be honest. If you are going to be getting a new 50" panny plasma and new speakers you are going to be watching a couple more movies a week, no?
You listen to a lot of music to, right and the price has to be reasonable. I think this is the perfect opportunity to grow into a tower system with the potential to upgrade in the future.
I am a bit confused though. You say budget isn't a factor but you want to keep it around $1500 so let me give you my opnion
If I were you and I have that current stoneage setup and have never had a taste of HT or premium quality speakers I would start myself out with the 4T hybrid system with the 4T's and pick up an Onkyo 605. That 16x12 room is a perfect size for a real world audition of this system. The 4B's are no slouch either! You are going to want a sub for movies, trust me. Also ask the Gurus if they can give you any special pricing on the Onkyo. I know they were talking about giving special package pricing on them one day soon. I'm thinking you can get everything under 2K.
Now, get everything setup. audition it for 30 days. Chances are you may be so blown away you will never think about speakers again. However, if you say you know I wish I got a little bit more from my center channel you could call and get a trade in on from a 4C to a 5C etc, etc..,. In the meantime you get a awesome complete system for movies and towers for some good 2 channel music listening...
Now if you are still unsure I would recommend picking up the phone and calling a Guru. They will get you the best setup at the best price possible that fits your needs. I know there are soo many choices and it can get overwhelming. So why not start small and if your not comfortable slowly work your way upward. I think it would be a much more enjoyable ride....and you can let all of know about those 4T's..You will be the most enviable person on here for being the only one to talk about having 4T's...I wish I had a pair!
Dan.