Newer game engines like the one in Oblivion need large amounts of frame buffer memory; when there isn't enough, performance can suffer. For the cards that have 320 MB of memory or less, the game crashes and returns to the Windows Desktop. The game uses many graphics techniques that demand hardware that can handle it. At the highest resolution, neither the 8800GTS 320 nor the 8600 based cards can render the game. However, if you could afford the 30" LCD monitor to play at that resolution, you would most likely have the money to buy a pair of high end cards to drive it in SLI.
All that said, the performance of the Foxconn 8800GTS is still impressive. Even at the torture testing settings we subject the card to, it can still be played at both 1024x768 and 1280x1024. The GeForce 8600 cards fall short of clean frame rates when there is a lot vertex and pixel shader work to be done.