800,000 actually seems decent for a traditionally slow quarter for the market when Nintendo didn't have many big games released anyway.
Mario Golf, FZero GX, and Soul Calibur 2 featuring Link should give the Gamecube a notable boost in the next quarter and, of course, a ton of Gamecubes will be sold in the final quarter of the year thanks to Nintendo's own Mario (multiplied by two!), Pokemon, Kirby, and the Pikmin with some help from some third-party gems like Viewtiful Joe, Metal Gear Solid : Twin Snakes, and Rogue Squadron III.