I thought that this link might help your brother make a sound decision. http://gear.ign.com/articles/720/720303p1.html
The second page has user reported lag for various HDTVs. Not a scientific sample but it might help.
You might also check out some local video stores and see if they have any big screen TVs hooked up to an Xbox or Play Station and give it a try.
I use my Samsung HL-S5687W about 50% of the time as a computer monitor. I don’t play action games on it yet but for all other computing including strategy games and video editing it‘s awesome.
Imagine going from an old 15 inch CRT to a top of the line 21 inch LCD. Now double that and it will give you an idea how much better the picture size and quality look to me. Supposedly Samsung worked with Microsoft to create a game mode on some models to enhance game play.
If I get a house and upgrade to a HD projector the DLP will be my only computer monitor, it‘s that impressive.