Cheap plasma's can only 480p - Not high definition. Sure, Wii can only do 480p, but if you are investing in a new tv, you would want it to be HD for future (and to an extent, current) tv content.
All decent sized LCD's can do at least 720p, which is (although not the highest) "High Definition". LCD's look grainier with low quality feeds and blacks look worse than plasma, but it isn't that bad and once the TV networks get better quality feeds, it will look better. You can get 81cm LCD's for $800-$900 at the big department store chains.