It's down to your graphics ram and system ram.
Systems with 64/128MB Video Ram and 256MB System Ram, will experience problems unless you put the settings of the game to match your card.
Rendering at higher resolutions is a peice of piss, for some reason cards can handle that no trouble. Running them with higher quality textures and models though will cause it to lag and jutter.
This is down to not only are the textures not stored in DDS form allowing them to instantly load, but also the fact that the game textures are general 1024x1024 in size.
On my FX5200 system w/256MB Ram (before I added some stuff to it) it loaded levels bloody slowly and ran with a jumping effect, until I turned the texture and model levels so far down they resembled Doom3 pixelation without the fantastic Dynamic Shadows.
I currently play games on a:
AMD AthlonXP 2600+
256MB PC2700 RAM
GeForce FX 5200 64MB AGP8x (3DMark 1,529)
Windows 2003 Server Enterprise
DirectX 9.0c
Forceware 67.02
Hitachi 160GB ATA/133
The system itself gets very respectable benchmarking speeds at 1088x612 .. Everything is set to default setting for it really. Apart from the train it runs alright.
It looks bloody aweful compared to what my 6800 Ultra is capable of. Even on the same damn system, hopefully when my new PSU gets here I'll be able to use it again. Fact is that Half-Life 2 relies very heavily on high-resolution textures to look good, and that is something that alot of budget cards are just not equipped to handle, either down to memory reasons or down to fill-rate reasons.