Well for to run HL2 decently, you'd need a 8 pixel pipeline card at most
Get either a 9700 Pro (2 years old) or a 9800 Pro, under 200$ each...
You can play HL2 nicely with either one, and they both have 256 bit DDR memory interface
The new X700 XT and Etc from ATI still use 128 bit even though it has DDR3 ram...
The more Bit memory interface.. well the more torque the game gets..
I can get a 9700 Pro for like 125-140 bux new.... you can probably get like a 9800 Pro ATI bult for 170 or under...use maybe.. also, you need a 300 Watt PSU... but if you don't have that,... get a 9600 XT video card.... which doesn't need a PSU connected to it..