What ever TV you get that is "flat panel" it will have a native resolution. If you get a widescreen TV that is 720p then its native res is 1280x720. I believe the wii does 720x480 in it's widescreen setting. Your TV must upscale the image to its panel size. A poor upscaler will result in a poor image, a better scaler will give you a smoother look, less jagged. Personally I'd go down in TV size if it meant I could get a better scaler.
Check out avsforum.com if you want a learn about HDTV.