I'd steer clear of the recertified card, it only has a one year warranty.
I would not advise using the twin 9600 setup. Even now, not every game will benefit from SLI, although most do, but I'd still prefer a single card over two low/medium end cards, it's more consistant and easier to manage.
Which of the others is a matter of personal choice: The 9800GT is the closest match to the CPU and still a fine card but it'll lack the horsepower to use max settings with AA in more demanding titles.
The 9800GTX would be my choice: It's a little cheaper than the GTX260 and has enough power to run AA/AF and high graphic settings in demanding games.
The GTX260 has the advantage of being able to apply high AA/AF AND run PhysX without suffering too much framerate loss: All those features are done by the card itself and will not be effected by the 'fairly' slow CPU. If yopu want PhysX and all the bells and whistles and enough power to have silken framerates in even demanding titles (CPU limitations asaide) then the GTX260 is the card for you.
BTW, which motherboard do you have? Have you considered a CPU upgrade?