Because you want to play these future games, I DEFINITELY recommend a video card that is DirectX9 capable. All of the cards mentioned above should work, so I would recommend the following:
nVIDIA GeForce line
Fx5900
Fx5600
ATI Radeon Line
9600 Pro
9800 Pro
Definitely buy a card with no less than 128 MB of memory, and if you have the $, opt for one with 256 MB.
In general, these products run neck-in-neck with their counterparts (I've owned products from both lines). HOWEVER, there is evidence that the ATI Radeon blows the GeForce line out of the water when running Half-Life2. This may be resolved by the time the game is released, but I definitely wanted to provide as much information as I could.
Check out this site for further reading pleasure:
http://www.tomshardware.com/business...r_nvidia_cards