Main thing is getting the right resolution and checking that its what it says it is. As with TVs many are sold as HD ready but are really 1024*768 which is nowhere near.
Genuine 1080p is expensive but the best.
As with everything else read the reviews before deciding.
You also have the choice of getting a proper screen or painting the wall white. All depends on if you are a penny pincher or not...
TBH I looked at buying one and then decided to go for a 47" 1080p LCD screen which is very nice - if you have the room