What's a good basic asset allocation? That's a simple question with a complicated answer. For help, I would suggest that the you read some books like Rick Ferri's All About Asset Allocation or William Bernstein's The Four Pillars of Investing. You can also read the articles at FundAdvice.com - Home
. All these places give consistent and similar advice on the subject.
In a nutshell: Figure out how the percentage fixed income you wish to have based on your risk tolerance. The rest goes into equities. For equities you have a couple of main methods of asset allocation: total stock market or slice-and-dice.
Even with total stock market, you have to figure out the percentage of domestic and international that you wish to have. World markets are around 50% US and 50% international, but some folks do not wish to go more than 20% international (I am not one of them). One can purchase just 2 funds with this type of asset allocation: a total US stock market fund like VTSMX and a total international stock index fund like VFWIX.
Slice-and-dice is more complicated. Some studies show a small cap and value tilt can increase returns and reduce risk over a total stock market portfolio. Throw in some REITs, a little overweight in emerging markets and maybe some other asset classes and you've got a portfolio with nuances that can be discussed for weeks on end and rebalanced occassionally for fun and profit. Read the books and fundadvice for more info.
For the fixed income, you can even divide that percentage up among TIPS, intermediate term, short term, etc.
It was not clear to me if the 403b is a legacy 403b and can be moved or if you are still working and must keep the 403b active. If you can move the 403b away from American funds, then I would suggest that you do so. First, the expense ratios will be lower at Vanguard. Second, you can select passively managed index funds at Vanguard that fit exactly the asset class you want. There will be no guess work about how the American fund divides up among the asset classes you are going for.
So what you need/want to do will take more work than a simple message board post. I hope though that this response causes you to explore further your asset allocation instead of just stopping here and not doing anything at all. Good luck!