well, we can save you a lot of time, Tim. You can't really integrate those things into a "screen menu" with a Mac. By screen menu, I'm going to assume you mean an "interface" or "front end" where you envision the Mac as central controller, the brains behind an overall system of separate devices which radiate out like spokes. The Mac in a home theater doesn't work well outside its own controlled environment--so your DirecTV and standalone dvd player and Comcast basic cable will stay outside whatever Mac and OS X-based front end you choose. Most folks who keep those devices and STBs end up controlling them with a universal remote, route audio through an AVR, and directly connect to their HDTV. And even if you get a kick ass Mac you'll still need a kick ass AVR.
Now, a Mac can take on some of those things but it usually is as an adjunct to them or an option: you can, as I said, play back actual dvds with your Mac, it does a decent job with them but not a great job, you can also play back video_ts files (the ripped dvd) which your standalone dvd player cannot.
Satellite is a closed system, you're pretty much out in the cold with that without hacking your box. Blu-ray and the PS3 also exist completely outside the Apple notion of home theater at the moment.
So what can you do? Music, of course. Macs handle this extraordinarily well, I mean picky audiophile "I'm ready to spend thousands because my ears are so fine" well.
With a third party add-on device like an EyeTV or Miglia tuner--plugged into a firewire or USB port--you CAN take that Comcast cable signal and tune/record/route through your Mac--and a core duo Mac has enough juice to do quite a nice job with high def. This will not be any of the premium channels, though--just the analog