plenty of cheap 42" plasma's are floating around, problem is some are pretty poor quality.
People are also falling for the trap of the price ie £999 for 42" - so lets go buy one! Only to find that they only have 1 or 2 scart sockets, no TV tuner and no bracket to install on the wall, which usually can cost up to £300!!!
LCD is better than Plasma, and is slowly catching up on screen size, however this is still at a premium as LCD is the thing to have, so if you can wait a year+ then do so and get an LCD, however if you have to have summat now and it has to be 42" on a budget, check out any reviews of the model number on the web and make sure the specification is what you need, as an example, I need at least 3 scart sockets and one AV input (3 scart for Sky, DVD and Video, AV input for digi camcorder) - or do what I did and spent £400 on a second hand £3000+ projector and a new screen and have an 82" screen instead

this pic is a comparison between my 32" screen which looks tiny!!!
currently looking at this myself though
http://www.comet.co.uk/comet/html/cache/182974.html
50" DLP (not LCD!) RPTV so slimline, plus its HDTV ready!!! to 1000 lines resolution!!!