MonkeyforaHead, FYI, "Thunderbird" is a specific version of the Athlon. AMD doesn't always advertise the different core names (such as Thunderbird, Thoroughbred, Barton, and a gazillion others). I'm not sure which core the 900 MHz Athlons run at, but if you've got a 900 MHz Athlon, it's almost certain that it doesn't support SSE.
While there is a compiler option to enable SSE optimizations, there are other ways they can get in - for instance, if Double Fine was using someone else's 3D engine that had some sort of assembly-optimized portions that used SSE. We can cross our fingers that they will release a build without it, if it is in fact the problem.