Not all index funds are created equal (expense ratio and what it is actually indexed to are very important). A lot of companies created index fund mutual funds as a marketing gimmick to gather assets (remember, they often charge 1% or more management fee on all those assets they collect, so they can make a lot of money, at your expense), but charge high expense ratios that defeat a lot of the purpose of an index fund per se.
If you have completely funded your Roth already, dollar cost averaging into
Vanguard Index Total Stock Market (
VTSMX) is a great idea because it is going to be very very hard to beat over time (say 30 - 50 years from now), and even more so on an after tax basis (turnover ratio is very low because it basically owns the whole stock market, so you don't have to keep paying capital gains every year). Plus, once you hit retirement age, you can take funds out of your IRAs first, and let this fund compound for another 10 - 20 years.
Expense Ratio (management fee and all costs to run the fund) for VTSMX is a paltry 0.15%, and because it is indexed to the whole U. S. Stock Market, turnover is exceedingly low. This minimizes trading costs and produces superb tax efficiency (you don't keep losing capital each year to pay Uncle Sam). Plus this fund is almost 100% stocks, where as a lot of actively managed mutual funds are typically 95% stocks and 5% cash. Over time, stock returns > bond returns > cash returns. Plus, if you have faith in the long term viability of the U. S economy, you can feel comfortable adding money in the most down of stock market days or dark days of recession since you won't need the money for decades from now. You do have to select a fund manager / fund company that you believe has your best long term interest in line with theirs, because if you have to pull your money out 10 - 20 year down the road and reinvest in another more trustworthy steward of your money, you have to pay capital gains then start compounding again with a lesser amount of principal. So fund company and fund manager you choose are very important to implement this strategy optimally (hint: VTSMX)
Two superb books to get you started:
http://www.amazon.com/Personal...&qid=1208832863&sr=8-1
http://www.amazon.com/Common-S...&qid=1208832883&sr=1-3
Perspectives from a successful active manager:
http://selectedfunds.com/pdf/SFSuccInv4Q07.pdf