I tried running an 8600GT and while I had no trouble hitting deadlines and such, the card would always screw up within a couple of days and force me to reboot the machine. Sometimes, it would cause EUEs within hours or sometimes it could go for a couple of days. There were no problems with temps or card instability of any kind. It seems that some work units just didn't like the card and caused the client to go unstable and screw up the drivers and display of the card. Everything would be fixed after a reboot but the same process would start over again. I also tried two different 8600GT cards and had the exact same problems with both.
Therefore, I don't suggest messing with an 8400GS. The points definitely wouldn't be worth the effort of messing with it as the best I usually got out of an 8600GT was around 1700PPD.
If it wasn't for the problems I mentioned earlier, I would have two 8600GTs running. That would add about 3k PPD on average minus what would be taken away from the SMP client to feed the cards. I don't really recommend running anything less than an 8800GT.