I think one of us is confused about the build-to-order options for the PB15. The faster superdrive model is the only 15" with the 128 meg video option. It already ships with 512 megs of system RAM and the only factory options are 512, 1024, and 2048 megs. There's no 768 meg option.
As far as which option to get, the best advice is to get all of the "now or never" options when you place the order and worry about the user-upgradable stuff later. You can always add system RAM on your own at any time. Unless I'm mistaken, you can't add video memory later. Even if you could, you probably wouldn't find the part for less than the $100 Apple wants.
The 512 megs of system memory that comes with that PB15 is plenty to get started. If you need more, you can always add it later. And much cheaper than Apple's upgrade options. 512 meg modules sell for $90. You can buy a pair for $180 and have a gig of RAM and two leftover 256 meg modules to sell on ebay. Apple charges $400 for this upgrade and you don't have any leftovers to sell.