A lot of times it depends on the work unit.
on my 460s for each card it varies wildly.
8018 =12,000 PPD
762? =25,000 PPD
7600 =18,000 PPD
So it is probably not your system unless your overclocks are not 100% stable for folding. A way to check is to run Afterburner and see what clocks the cards are running at.
If they are not fully ramping up to 3d performance clocks then it is not folding stable.
You will need to reduce clocks or increase voltage to reach stability.