If you are using an external game machine and TV tuner, such as a Set Top Box from cable, SAT, etc you can use an HD monitor rather than a full TV. Monitors tend to run smaller, but less expensive than full HDTV sets.
How large depends partly on how far away you will be sitting. If you get a 48" screen, and sit 24" away, you won't see the entire image without turning your head, and that can get quite tiresome after a 2 hour movie.
I just purchased an 24" wide screen HD monitor for my computer for $250.
It does not have the TV guts as I intend it to be mainly a computer monitor. I will be connecting the STB from Verizon FIOS using the HDMI connectors, so I will have HDTV and the computer on the same monitor.
I will be only about 24" from the screen, so 24" screen is the perfect size.
FW