60-80% of any game's budget today is artwork.
5-10% is programming (AI behavior, pathfinding, saving games, UI, etc)
There will be soon enough another technology generation with new 3D cards and DX 8 will become DX 9 or DX 10. I'd suggest completing as much as he cares to before that technology change makes continuing development with SS a pain in the rear, (do the movie work to clear the pallette), then make the sequel on the next generation of development tools, using what's been learned from SS and having some time to get away from it and think about it with some distance.
Jim's commented he feels competition breathing down his back previously, but that's just not true. All the other Aquarium programs I've seen were by people who knew the 5-10% programming part so-so, but were completely clueless on the far more important 60-80% artwork side of entertainment software. The competition's artwork is uniformly amateurish.
I *would* look into using some better software tools like
http://www.perforce.com which is free with a two user license and is excellent for organizing and archiving painlessly the many different revs of artwork he goes through (I know one publishing houses uses it as their art group evolves their book covers) It saved my butt when Melissa destroyed my dev machine.
