I'm looking at getting the Vizio vx42l, it says it has Integrated NTSC/ATSC/QAM HDTV Tuner. Does this mean i still would have to purchase a box from comcast to get hd channels, or do i just have them upgrade my service to hd?

I'm also trying to figure out/find a dagram for wiring the system.
It includes:
tv vizio vx42l
dvr tivo series 2
receiver rca rt2600
dvd jvc xv-n50

Any help would be appreciated.

You should be able to get your local stations in HD without any box and any channels that are sent out in the clear, that means unencrypted. If you do choose to get the cable box from your cable provider, you will lease it from them, they do not sell them in the US.

You'll need a splitter - the coax from the wall would go to the splitter and from there to the TV and Tivo. The video output of the Tivo goes to the TV while the audio output goes to the reciever.
The DVD is the same as the Tivo - audio to the reciever while the video goes to the TV.
You can watch cable directly on the TV or via the Tivo, but only the direct connection will get you HD.

You don't need to get anything from the cable co if you are satisfied with what's already available. Check your local HDTV threads on what's available in clear QAM.