What is a reasonable number of draw calls relative to one hardware configuration ?

The project I was working on recently managed just under 2000 draw calls on a 6800GS 128MB running at 1280x1024 resolution. The only thing that bottlenecked it was when the texture memory required rose over that 128MB mark, and obviously the card couldn't load everything in at once. That was our minimum target graphics card for what we were attempting to do.

At 400 draw calls, you shouldn't be stressing the system too much, but it all depends on what graphics cards you intend to target. Integrated GPU's with shared RAM may have issues with this level of draw calls. My best recommendation is to pick a target low end platform, and make sure your game runs smoothly on it at a decent resolution. You could pick up the parts to build such a system off eBay quite cheaply.

You should also note that Draw Calls are actually a bottleneck largely dependent on the CPU. They are generated each time the CPU has to send data to the GPU for processing. GPU's are very good at processing large amounts of data very quickly, but they are not good at changing what they are doing, hence why batching work into larger units can benefit your performance greatly. So don't apply all your testing based solely on GPU's, as CPU's have a greater effect when your performance is Draw Call limited. If it is fill rate limited, then it becomes a matter of the power of the GPU.

To determine if you are fill rate limited or draw call limited is simple. Run an FPS (Frames Per Second) monitoring program in the background like FRAPS to monitor your performance, and play through a sample of game at your highest available resolution, then again at a much lower resolution. If you game performance changes dramatically, then your game performance is fill rate limited, and you should look at the shaders you are using for better performance, along with the GPU. If the performance change is minimal, you are draw call limited, and should look at reducing the number of draw calls through batching or lower the number of objects visible at any one time.