According to a post on Russian website iXBT, who quotes Chile Hardware, a 256-bit version of the GeForce 8600 GTS might arrive in June. The card would feature the same clock speeds as current 8600 GTS cards, but it should have 512MB of GDDR3 memory.
This is purely speculation, but iXBT has posted a spec sheet, although this might not come from a reliable source, especially when you consider the English used, as it says "dangle" instead of dongle for the VGA out on all three cards listed.
However, this could information from a Taiwanese board partner and then the spelling mistakes are understandable.
If this information is correct, then these 256-bit 8600 GTS cards should appear during Computex.
You can find the original article here in Russian.