There is no special chip that makes any display HD. As others have said, HD is a standard resolution to which HD Movies are produced, and that standard is 1920x1080p. You appear to be confusing TV standards with LCD monitors.
There are two requirements for a TV to be considered HD-Ready.
1. It has a native resolution of at least 1280x720 and must be able to display at least 1280x720p movies.
2. It must have a DVI or HDMI connection that supports HDCP for content protection purposes used in HD-DVD, Blu-Ray, HDTV Broadcasts, etc.
When it comes to buying a monitor for your Mac, almost any monitor made in recent years will likely meet requirement 1. Requirement 2. will only matter if you want to be able to plug in the HD output from HD-DVD, Blu-Ray, or Cable to the monitor.
As to the question of what to buy for HDTV, Xbox360, HD-DVD, and so on. A TV. Not a monitor. Most monitors have an aspect ratio of 16:10. TV broadcasts, Xbox360, PS3, HD-DVD, etc all have an aspect ration of 16:9. If you try running these on a 16:10 monitor then they tend to look a little stretched and wrong. Monitors also tend to be optimised to display text and graphics where as the the TV tends to be optimised towards a natural picture for movies. TVs will generally have all the connectivity options you require (HDMI, Component, and occasionally VGA) that most monitors don't provide.
So if you plan on watching lots of TV, Movies, and playing Xbox360 or PS3 then you really need a nice TV. Sony Bravia's are nice by the way.