I have an older Sony trinitron from the early 90's and was thinking about upgrading to a used HD XBR800 model television. The problem I know from playing video games like Rock Band on HD RPTV's or HD Plasmas is really bad lag. Even when it is calibrated in the game, it still seems off.

Will I experience lag playing xbox360 on a Sony 36xbr800?

I play 360 and PS3 on my XBR960 and lag is non-existent. You should be fine with the XBR800.

I had a 34XBR800 - absolutely zero lag with Halo 3. My new LCD with VGA seems about the same.

I agree, no lag on CRTs. You must have tried some early model projections & plasmas to be seeing some lag.

Ive done a lot of gaming on my XBR960 CRT HDTV and never had issues with lag (Xbox 360 / PS3 / Wii) Same goes for my Pioneer plasma.