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The GOTY version comes fully patched, and with the smallish add-on "Knights of the Nine" (extra quests and items). KotN is also available online for download, and packaged with other Official Mods in a boxed product as mentioned on the bottom of this page: http://www.uesp.net/wiki/Oblivion:Official_Mods

IMO, if you want to buy just the original first (to check out the game / modding) then possibly buy SI later, missing KotN content is not a super major loss.