Dunno’bout Nvidia (as their developer-page is much huger as ATI’s, I guess they have such a thing too), but ATI have a document called “Radeon 9700 OpenGL Programming and Optimization Guide” which should be what you’re looking for (at least for the R3x0-architecture). It’s 13 pages long and is very informative for everyone developing on that kind of HW. It tells you for example what shader instruction need what resources, what datatypes are supported, and so on.
You can find it in their (rather small and else disappointing) GL-SDK.
Edit : Here’s the link to the GL-SDK …their developersection is a real mess.
But really, if you’re a developer, purchase as many different cards as your QA budget can afford. If you’re pushing the hardware, you’ll run into different performance characteristics for each card and driver bugs.