As others have said, the only real option you have when buying a new television today is probably the Sharp Elite, and I don't know that I'd consider it an upgrade. (better in some respects, worse in others) Certainly none of the Plasmas available today are better than what you have.
If you actually care about image quality, you're better off spending that money building a gaming PC, because console image quality sucks. Next generation will look better, but you're still going to have developers cutting corners on things like resolution, texture filtering, framerate, and antialiasing
I would expect most next-gen titles to still only be running at 30fps on consoles (compared to 60–120fps on PC) and the majority of titles will probably start utilising blurry post-process AA techniques like MLAA/FXAA/SMAA, which look awful and can be forced off, on PC. (on PC you can use proper MSAA or supersampling—at least if you're using Nvidia)
A new TV will display the console image as best it can, but a gaming PC will make the image being sent to the display a lot better, and I think that's going to make a much bigger difference than upgrading your TV.
Looking forward, there's 4K displays and OLED which have the potential to be a real upgrade over your current TV, but they're going to be expensive, and in the case of 4K, the consoles aren't going to be rendering games at that resolution anyway, just like almost no console games are rendered at 1080p (or even 720p) today, whereas 1080p is trivial for a gaming PC.