If I recall correctly, the adjustment for natures was 10% of the base stat value. Also, I seem to recall that characteristics reflected the highest IV only, and didn't actually apply a separate increase. At any rate, you'd want a characteristic that reflected a high IV in the stats you most want to emphasize. Most likely this would be either attack, special attack, or speed, but YMMV.
The reasoning behind this is that natures are kind of a game of min/maxing. You sacrifice your abilities in one area of hopefully less benefit to the pokemon in question in order to increase your abilities in another more useful area. So, say you have a special attacker. You get a nature that decreases his physical attack and increases his special attack, and then you load him up with special attacking moves. It wouldn't make much sense to want to add points into his physical attack if you are never going to put a move that uses said stat on the guy.
No problem. Oddly, it was when my friends got me to pick up Pokemon Colloseum that I really became interested in the nitty-gritty of the game mechanics. But all those little tweaks can really make a difference.