Funny, I'm running the game smooth as butter at either 1280x1024 w/ 4x Anti-Aliasing or 1600x1200 w/ 2x Anti-Aliasing (take your pick) on a 3 year old 2.26 GHz P4 OC'd to 2.76 GHz and a Geforce 6800 GT...
It's definitely running slightly smoother than HL2 although you're right, it still doesn't look quite as nice.
Do you actually have any proof that the game is "hogging" system resources or are you just saying that because your frame rates suck? This could be caused by any one of a number of issues though my own experiences would seem to suggest that an unquentiable thirst for raw processing power or system memory is not one of them.
Perhaps you are suffering from one of the 'minor issues' noted in this
post from the game's lead programmer. Also check this
Are you sure that all of your drivers are up to date?