Believe in the S&P! According to Bloomberg Wall Street strategists need the S&P 500 to rally 20% by December 31st in order to realize their forecasts.Analysts at Goldman Sachs Strategas Research Partners and JPMorgan Chase are some of the most pessimistic but still say stocks are cheap relative to earnings - and an easing of the credit crisis could help boost equities. That kind of gain in the S&P would mark the biggest year-end rally on record. And given the recent volatility it isn’t totally out of the question. However as it stands the S&P 500 is still poised ...