Thanks, all of you! The links were great, and led me to even more information out there (much of which I didn't understand, but alas...) I think from what I'm reading, at my weight and performance level, it probably doesn't matter much whether I go with 25s or 28s. I'm leaning toward 28s then just for comfort, if I can find a good tire I like in that size - from what I'm reading, it appears the high-quality road tires don't often come in 28. If my performance really gets up there I may switch back to 23s or 25s for racing. One of the things I came across suggested that 28s would perform better than 23s at the same tire pressure, but obviously you wouldn't pump 28s up to where 23s would be...would it hold then that I would want my 28s at their max since I'm riding my 23s higher than need be for my weight, or would I want the 28s less inflated also? Clearly I have a lot to learn!