Looking at a WoW box, a BC box, and this, the published space requirements were 4GB, 10GB, and 15GB respectively. Expecting the data directory to grow to 13GB is not unrealistic - although I suppose we'll know for sure in a couple weeks. Also note that it will grow a little thanks to content patches etc, which this may or may not already allow for. I think it does, since my WoW+BC install directory reads out at 9.36GB.

Right now, it would be unwise to do with 12GB since that leaves you somewhere under 4GB to play with. With Wrath, it may be out-and-out impossible. Even 16GB may not be quite enough with Wrath, but we'll have to wait to find out for sure.

You can always just get 8GB of the 1066 and scratch the RAMdrive idea.