When is it plagarism

Bunsen,

Here are answers to your questions, but the first question is whether you actually want to make a product that uses the D&D/d20 rules. If not, then this becomes a slightly different discussion. If you do want to reuse game mechanics as well as names and concepts, here are answers about the Open Game License:

A) Yes, everything you find in the SRD is guaranteed to be Open Game Content. You can re-use any piece of Open Game Content as long as you print the Open Game License in your book, and name each source you borrowed OGC from in its Section 15.

B) You can copyright the entire package of your work, even if some parts of it are "borrowed" and others are "copyleft" (in otherwords, the OGL allows sharing of otherwise copyright material).

You must designate parts of the work are Product Identity; in other words, those bits which you created and aren't derivative of other OGC. A common rule of thumb is to designate everything that's a mechanic or rule as OGC, and to designate everything that's a story, proper name, setting element, and other flavor/color bits as PI. You can decide to make some setting bits OGC, but it's not recommended to try to make some rules PI unless you're very sure they wouldn't be considered an extension of the basic D&D rules.

You can trademark individual words & phrases to add an extra degree of protection/control over them.

Check out the official d20 FAQs herefor for a pretty good guide to these issues.