If you are going to buy a new tv, get one that supports all the resolutions! That way you are never going to run into resolution problems.
These are the key features for a new tv.
No Lag
480i, 480p, 720p, 1080i support (no converting is better, up or down converting lowers the quality, and my produce lag)
Clarity from different angles. (some tv's only look clear and bright from a 75-90 degree angle (straight on or slightly to either side) Depending on the layout of your room you prob want 50-90 degrees or better.
Brightness (more visibility when there is light in the room.)
Contrast & Overall image quality (this is obvious, just looks at a few tv's while you are at the store)
Also, if your are buying a new hdtv I assume your are buying a total entertainment system. Don't buy anything but the tv at the store! They will rip you off on all the accessories, and cables. If your are buying at a store that offers commission to it's employees, remember to negociate the price, the sticker price is atleast 15% higher than you can get it for.
DON'T BUY A THIN PLASMA/LCD unless you actually need to save that much space. LCD/Plasma is lower quality, and is more failer prone. Buy from a decent manufacturer. I recommend Sony, and Mitsubishi. Other manufacturers like Samsung, Magnavox, JVC, ect make great consumer electronics, but they are new to the hdtv market, and this means that they will have many more issues than the Sony and Mitsubishi tv's.