One word....pointless!
Only games at the moment which need 512MB on the graphics card is Doom 3 at Ultra Settings, and no 6600GT or 6600 can play it decently with that setting. Most games have been adjusted to need more system RAM than graphics card RAM, the latter makes no difference apart from on some ATi cards, mainly the X800s (128mb is much slower than 256mb for some reason)
Only games at the moment which need 512MB on the graphics card is Doom 3 at Ultra Settings, and no 6600GT or 6600 can play it decently with that setting. Most games have been adjusted to need more system RAM than graphics card RAM, the latter makes no difference apart from on some ATi cards, mainly the X800s (128mb is much slower than 256mb for some reason)