40' is fine for a regular HDMI cable. Don't go crazy and buy a super expensive one. A 50' high-speed HDMI in Canada should run around $70 or less. I would also look into an Asus EeeBox with HDMI to mount behind the TV if you really did want to mount a computer there.
HDMI over CAT5e or CAT6 is great, but expensive and not necessary for the length of your run.Attachment
