Great question Christina and one that a lot of people wish they had the answer to.
I, for one, believe in the incredible growth of areas South of Market. With all of the development in the area including new homes, park lands and recreational spaces, new companies, and potentially a new school, SOMA and all its neighborhoods are sure to increase in value. When you add in how convenient transportation is in this area too, this seems like a sure bet when it comes to a new home or an investment property.
Other places to look into may be the Tenderloin which is a historical area and will only get nicer with time (think Harlem in NYC). It may not look perfect in the next 5 years but you can bet that being that close to downtown and Union Square is going to help in property value in that time frame.
I hope this is helpful and feel free to contact me anytime if you want to get into more specifics.
p.s. See my blog on SOMA at: http://www.trulia.com/blog/gabriel_rojas/2011/12/south_of_market_-_the_future_of_san_francisco