The GoldenEye on XBox Live Arcade
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Even now, rare's N64 classic Goldeneye is concidered one of the greatest FPS's on any console. Mainly because of the multiplayer. Most XBL Arcade games are small classics, like Street Fighter and Doom. However, I'm sure a decent port of Goldeneye would not take much time to pull together, and would surly quickly become XBLA's biggest hit. I'm sure most people would also be willing to pay more then the usual 800 points for a Goldeneye download.
...Think of the money.