The two VGA card are very close in power. you can not really tell while playing games.
Myself own a G73jw-a1 my friend just recently baught a GT660 the actuall 3dmark06 score only has a 500 difference. (G73 a little higher)
the main thing you have to think about is the resolusiton of the screen. G73-a1 is full HD, but the GT660 isn't.
there is a lot of different versions of G73 and there is also a GT660r so make sure you read the details carefully.