LCD or Plasma.
Depends what programmes you like to watch. If you are a sports or movie fan, definately plasma. General TV, probably LCD, although personally i would still choose plasma.
Plasma tends to give a warmer, more natural picture, with greater depth. LCD tends to be brighter, due to the fact it is backlit as opposed to self illuminating.
Biggest LCD is about 47 inches, plasma 63" plus (the plus is at silly money!)
HD. 'Full' HD sets are 1080p, ie 1920x1080 lines, as opposed to 1080i, which are generally 1366x768 lines, and give a fantastic picture, but remember that you need a full HD feed to get the benefit. By that, I mean Blu-Ray DVD (or HD DVD but HD-DVD now has a very limited lifespan due to Toshiba pulling development in this format.
Sky HD, and Freesat HD are only output in 1080i.
Hope this helps, please post any other quesries and i will do my best!