I'm pretty happy with my garage setup. Two comfy chairs, the old 32" box and stereo system wired for surround. A little space heater takes enough of the chill off when it's really cold out. As far as ventilation, if I'm smoking alone I just crack the window and the back door for a little flow. If the wife and/or friends join me smoking, I run the garage door up about 6" and open the window, then leave the window open overnight to clear it out.
I'd guess the easiest way to run your cable would be to find the access point into the house and just run your line on the outside of the house to your garage. Our cable access was actually through the garage so I didn't have the problem of running line.