tedster - 10:28 pm on Mar 31, 2010 (gmt 0)
Most of the time, a good ranking depends more on backlinks (plus title tag and anchor text) than it depends on the basic HTML text on the page. If you don't touch those more sensitive areas, then small tweaks to the basic text are usually just fine. New images are usually no big deal either.
However, if you plan to make major edits, or to expand/reduce the amount of on-page text to a significant degree, then a good ranking can go wobbly. Or it can improve, too - it's just that you should recognize that you are into a more sensitive area.