I'm actually also using a GTX 260. Thanks to harvy for spurring me to tweak the settings. With the new cudaminer build and some more aggressive settings than the ones previously recommended in the readme, I'm getting a few more kH out of the card.

What are your config settings? I'm thinking cudaminer might start loading CPU if it's set to use (GPU) devices that don't exist (and other strangeness).

I was also having some issues running both cores on pooler's cpuminer at the same time as cudaminer with stability, so I've stopped doing that since it was a minor increase anyway. On my second rig I leave one core free for general browsing and stability, which seems to work fine with that more advanced GPU (GTX 670MX).