Nvidia cards perform much worse than ATI cards for the kind of work coin generation is doing.
For a good breakdown of what rates are typical for various setups see: http://pastebin.com/AvymGnMJ
From the second link it seems a 260 should be able to pull 30-50 MH/s with the various settings listed there.
For windoze, MSI Afterburner software is a free and easy tool for overclocking the core clock and underclocking the memory clock.
Looks like core clock > 1200 and using the "-w 128 parameter" on the miner should be the sweet spot for that card.