I would shoot for one 9-cell battery and one 6-cell. That's what I did and it's great! I use the 6-cell around the house or in buildings when I can plug it in after a while and the 9-cell when I'm travelling.
Anyone know when Notebook Hardware Control will be updated to include Merom compatibility? RMClock has a nice forum and I asked the programer and he said that it could be next week...but I like the interface on NHC.