There aren't many manufacturers that make 256MB 6600GTs but they're definitely out there.
This one for example. But you are right they are few and far between and the companies who do manufacture them make very little, which is why I said 6600GT at a minimum.
And as for Doom III (and I assume Quake 4 as it uses the same engine), it has single textures that are >512MB fully uncompressed. If you really want to find out what you can live with in terms of hardware performance for Quake 4 I suggest you read
this. It should give a general idea of Quake 4's performance on various hardware.