i run battle field 2 on my 1.83 mac book pro with only 128 mb vram and it runs fine, most settings on high, textures on medium (you should be able to set textures on high)
i have 1.5 GB of ram, that helps.
what game are u talking about ?
what kind of lag do you have, regular, or choppy ?
all maps ?
if the lag is irregular, it's probably caused by the game swaping in/out parts of the map.
if the lag is constant, and gets worse when more things happen on the screen it's the video settings, take a look at them.