Assuming the memory is running at the same speed, the 256mb version is marginally better... but not worth paying more than $15 dollars for.
More than likely, the 256mb version has inferior RAM though, and that will affect performance much more than the amount of memory, especially if you plan to overclock...
<b>Geforce <font color=red>6800 Ultra</b></font color=red>
<b>AthlonXP <font color=red>~3300+</b></font color=red> <i>(Barton 2500+ o/c 412 FSB @ 2266 Mhz)</i>
<b>3dMark05: <font color=red>5,275</b>