I didn't know a 9600GSO came with 256MB (typically 384MB or rare 768MB). But regardless, Avoid a 256MB version of either IMO unless you game at 1280x1024 without fsaa or lower. If the price difference is huge, then they may be worth considering, but with such cheap 512MB 9600GT, 8800GT, and 9800GT, I wouldn't look too hard at the 256MB ones. The 384MB 8800GS or 9600GSO isn't too bad, and can be priced dirt cheap ($50 AR shipped in USA). There were some games at 1280x1024 with fsaa that the 256MB 8800GT (one step above 9600GT but slightly lower clocked than 512MB version) just tanked in performance, so it's not always just high res.
9600GT 256MB vs 512MB:http://xtreview.com/addcomment-id-4704-view-GeForce-9600-GT-256mb-vs-512mb-benchmark.html
8800GT 256MB vs 512MB:http://en.expreview.com/2007/12/30/review8800gt-how-many-memory-do-you-need%ef%bc%9f.html/16