The lack of 6 pin connectors could be an issue, depending on other factors.
1. What is the total 12v rail amperage of your power supply?
2. Do you have any available 4 pin molex connectors? (If so, how many?)
They now make 9800 gtx+'s that only require one 6 pin supplementary power connector. Thus, if you bought one of these, have 2 4 pin molex connectors available on your power supply, and have enough amperage on the 12v rail of the power supply, then you could SLI.
Toms has charts for performance gains...http://www.tomshardware.com/charts/gaming-graphics-charts-q3-2008/benchmarks,30.html
With the answers to the above questions provided, someone else can give you a definitive yes or no.