1-Is it still best to record Audio files to an external hard drive?
2-Can I record audio safely to an internal drive? If so, what internal drive (type, company that sells it, etc.) do you recommend? There are such divergent opinions! -Will there be any potential problems if the internal system drive has 8MB cache and the new 2nd internal drive has 16MB cache?
3-What's a good size drive, internal and external for audio and video?
4-Do I have enough RAM for all this with Sculpture and Space Designer?
Here is my personal opinion on your questions.
1-It is best to record your audio on a different HD than your main system one's. It can be an internal or external HD.
2-Yes you can. An external HD can be useful because of portability.
3-Naturally, the bigger the better.
4-Yes you have. SD and Sculpture takes up much more CPU power than memory. Even then, If you plan to use a lot of samplers, 4.5 GB is excellent. In fact, until Leopard and Logic 8, Logic can't even access more than 4 GB (and in practice, even less).