TV reception and crispness on computer monitor

I've always had this concern with my computer monitors over the years. I use my computer for gaming, cable-supplied TV viewing and recording sports, word processing, photo processing, DVD playback, the web. While the color crispness and sharpness of images is excellent when playing video games and all other tasks, computer TV quality is quite lacking when compared to a Sony 1080p, 1920x1080 native resolution TV set in an adjacent space. I could be viewing the same TV show and the standalone Sony is remarkably better in picture quality than what's on my monitor.

It concerns me that when seeing the disparity in quality between the monitor and TV set, I think of the price of my $1300 Samsung computer monitor and its $550 graphics card, whereas this TV monitor cost me about the same as the 30"Samsung computer monitor. I assume I hooked up and adjusted my computer monitor correctly for TV reception through Windows Media Center. What gives? I know that I'm probably missing some big consideration. Thanks for any input you can offer.

Asus Rampage Formula MB
Q9550
8 gigs Mushkin Ram
HD Radeon 4870x2
860 watt PSU
Hauppauge TV 1800 tuner card
Vista Ultimate 64 bit
30" Samsung monitor
Comcast standard cable service