Yeah, 403b and 457 accounts on top of 401k is an awesome combination, please take advantage of those (I wish I had the option!).
There were two things I wanted to point out just to be sure that you have everything straight.
First of all, just understand that there is nothing all that magical about "just getting into" the 15% tax bracket, your taxes will be almost identical whether you are $1 below or $1 above, some people misunderstand the idea of marginal rates, etc. And the same goes for qualified dividends and long term capital gains, if your salaried income minus deductions and exemptions (before dividends and cap gains) just barely puts you in the 15% tax bracket, you're not going to pay $0 taxes on those dividends and cap gains once you add them in. In other words, your salary plus those needs to all be within the 15% bracket. But even if just a portion pushes you over, you only pay the capital gains tax rate on only that portion.
Second of all, you mentioned something about in a great market year you might see a lot of gains, etc. I just wanted to point out that when dealing with mutual funds at least, there are two different kinds of gains that you can realize. First, the funds have to trade throughout the year to some extent, and while many of the low fee index funds are good about avoiding these, the funds themselves may realize capital gains that they will pass on to you. You will have to factor these into your taxes for that year, but again it's typically small or zero, and only relates to holdings in after tax accounts. However, you do not pay any taxes on the actual appreciation or "capital gain" of your stocks/bonds/mutual funds until you sell something and actually _realize_ those gains. So in your accumulation phase, you will probably do little or no selling at all, and while the value of your holdings will rise, you do not pay any taxes.
My apologies if this is already all known to you, but I just got a hint that maybe you didn't.