The worst way to put space between paragraphs is to insert an extra carriage return. (See carriage returns if you’ve forgotten why.)
As with first-line indents, you want the space to be large enough to be easily noticed, but not so large that the paragraphs seem disconnected. A space equal to 50–100% of the body text size will usually suffice. The larger the point size, the more space you’ll need between paragraphs to make a visible difference.
Is space before a paragraph equivalent to space after? Pretty much, though I prefer to rely on space after, and reserve space before for special circumstances. For instance, a block quotation may need space before and after to look vertically aligned.