I always liked half-life, they did a good job on it (though in the later years I've found that it makes me sick to play it, Freeman runs too fast, and while I switch it into walk-mode to make it better most of the time (when I don't need to run), I still found it to be a pain on ladders. Comes from playing too much Theif I guess (there not only do you almost never run, you spend quite a bit of time crouched and creaping along
HL2 looks killer (if the screen shots can be believerd), though I wouldn't hold out too much hope of playing it on too aged a PC. Thankfully it's going to be out for the XBox as well, and while consoles are't the best for FPS games, it'll beat the long wait till I replace my very-aged PC (it feels funny to say that with a 486PC with a 586 Evergreen upgrade to 100Mhz or so and a VESA LB SVGA card in it sits in my closet!). Article I read (I think in EB's GMR magazine) said that the XBox version doesn't look as nice as the PCs... this tells me the PC version is running way over the top of the XBox, which is still fairly cutting edge.
BTW - have you all seen the facial animation stuff they'ved added to the engine? It borders on creapy!
PHIL