Basically each game depending on the complexity will run at a certain amount of frames per second. You have a Gefroce 6600GT which is a very good card and will run new games at rather high resolutions at a good frame rate, optimum for playing games.
Newer gasmes of course have more complexity and frames per second ist he reason why people get new cards. That and new direct X versions and so on.
The command console can be used to find out in mid game, though it doesn't really matter to be honest if you are a standard gamer. If it is smooth and your noticing no trubles and slow down then you know it is going at least above 30 fps, and so no need to worry.
On the card you have, you'd excpect half life 2 to run at the range of 60-80 fps? maybe even higher on inside scenes for example. Definetly good enough
Hope this helps