For not only years, but decades, tracking the S&P 500 has been a reliable way to generate significant stock gains. Since the index tracks the best stocks on the U.S. markets, it offers a great low-risk way to ensure you're positioned for long-term growth.But what if you invested a lump sum of $25,000 into an exchange-traded fund (ETF) that tracks the S&P 500, such as the SPDR S&P 500 ETF (SPY -0.07%), and simply held on for 25 years? Could that be enough to make you wealthy and allow you to retire comfortab ...