Having now seen the trailers for Half Life 2, Doom 3 and Halo 2 I can happily say that they all look dead pretty, but Half Life 2 is going to be the one I drool over the most.
Doom 3 looks insanely good - but is very glossy. It looks like it'll be a fun, frightening 1st person scare-em up.
Halo 2 doesn't look as pretty, and seems to be basically Halo 1 with extra nobbly bits. Nice nobbly bits, to be sure, but I didn't see anything I couldn't live without.
Half Life 2, on the other hand, is amazing. The graphics don't look as polished as Doom3, but the animation looks more lifelike, the engine looks more flexible and the whole package just looks unbelievable. Watching a demo where they shoot out bits of fence and have it fall naturally into pieces where you shot it, or having rubble bouncing off of things following the laws of physics, or people lip-synching perfectly to the dialogue or watching a video camera being thrown about the screen while several displays show the view (upside down at times). Very, very impressive.
You can get the trailers here: Half Life 2
You'll need Bittorrent to download any of these, but I recommend you install that anyway, because it's a fantastic downloader for large files
(if you're interested in why, see here
If you have a problem trying to download any of them - right-click and save the file, then open it up from your computer. That seems to force it to work.