I've got a 42" plasma that has some significant burn in from PC use. Activating the pixel shift and color inverse options helped a bit, as did running snow and gray screen for extended amounts of time but it didn't totally eliminate the images.
I suggest that you look at LCD for gaming if you're concerned about burn in. Note that LCD's can get a burn-in effect called image persistence
but is temporary and usually easily fixed.
I just got a 52" LCD for USD$2200 so the prices are coming down. You may be able to find your 50" for 2 grand or so.