Yes, totally agree with s2.0, i know many friends who just can't afford a new card with s3.0, let alone 4.0 or 4.1 support.
Shader 3 - that's DirectX 9, right? That's the GeForce 6 and 7 series - you should be able to get one for $20.
But hey - if they support shader 2, that's great . The more systems it supports, the better.
AI would be a thread. We just got graphics into their own thread. Other libraries that we use are already using their own thread, so as the number of processors increases, the game should scale automaticially.
Well, a possible issue with this method is that some threads may barely be using the cores, while others might saturate the core(s). You may get uneven usage, and if there are a lot more threads than cores, overhead may become an issue.
For maximum performance and minimum overhead, the ideal number of threads is the number of cores the system has, and each thread should have the same workload as the other threads.
There's an intresting article at Ars Technica about how Valve decided to takle multiple cores. Might be worth a look.