US Public Interest Research Group has released two reports that might be interesting:
Contribution Limits and Competitiveness: an analysis of how state campaign finance laws affect challengers and incumbents. The bottom line, according to the report, is that there is negative correlation between the existence of contribution limits and the margin of victory for winners.
The Wealth Primary: The Role of Big Money in the 2002 Congressional Primaries. Large contributions from few donors gave winners an edge. The winners usually had far more money than losers.