I would say not a good idea, I have a 43" Pioneer Plasma (PDP-433HDE if your interested) which sports True XGA resultion (1280x768@60Hz) 😀, and it's nice to use as a 2nd display with games, DivX movies etc. But I would not want to use it for general use, e-mail, surfing, photoshop blah blah.
If you have the cash to buy a Plasma screen though, then I would imagine you could spare a couple hundred $/£/whatever on a decent 19/21" CRT for day to day work, and then get the Plasma aswell, then you can have the best of both worlds 😉