Half life 2 will be playable on most cards reasonably well.
It will depend on what kind of card it detects when you install the game and will load the appropriate settings for each card.
For example when you scope on a far away object you'll see it fine, but the detail around will be scaled down a bit on a lower end card, but it will play almost as fast.
Each card will have it's own advantages in this game.
Higher end cards will get all the eye candy.
Most features will be playable down to a G-Force 3, but with a limited set of functions., as they don't support the shaders that are in directx 9.
Directx 9 cards will display the most eye candy, it will depend on how fast your card is for the framerate and resolution choices.
The more features you have on your card the higher it will scale as in a Radeon 9800Pro or GF- FX5900 ultra
PC Gamer did an article about it in their last issue.
And surprisingly most of the cards did well because valve scaled the game accordinly to what your using.