I always use relative rather than absolute sizes, as felgall says they maintain the proportions. A gap of 5px might look right at 12pt but be too big at 8pt and too small at 18pt. Relative sizes are recommended by the Web Accessiblity Guildlines (WAI) partly because they adapt to display on other devices better.
I agree with AlexDawson that appropriate mark up should be used, again this is something encourage by WAI. A paragraph tag, <p>, would seem to be more appropriate in this case.
There is a behaviour in browsers which I believe is called "margin collapsing", where if one paragraph has a bottom margin and the next paragraph has a top margin then the gap between them becomes the larger of the two margin. I wonder if this might be ChrisjChrisj's problem.