Please help me decide.

I currently have a Sony 36" XBR CRT. A neighbor is selling his 40" XBR for a 50" LCD/Plasma. What would be a fair offer on my part since he said he would sell it anywhere from $500-$700 including the stand which cost $500.
I wanted to aquire a 57" DLP, but would keep the 40" for about a year until I bought something else.