No.
Apple never went beyond the prototype stage of a Gigabit card for the Cube. As such, there are no Gigabit cards or upgrade options available for the Cube. There's a picture of the prototype in the gallery (photo 1 | photo 2).
PowerLogix has stated publicly many times that the cost of producing such an upgrade for such a miniscule market assure they will never ever do it. No one else will likely ever bother with it either as the market for it is very small and the costs involved with producing it are very high, making the profit potential nonexistent.
To answer another person's question: No, you can't just take a Gigabit PCI card and substitute it for your Graphics card. The graphics card slot (with or without the AGP riser) is not like a normal PCI slot found in a tower, and as such, you can't just drop a standard PCI card into it and expect it to work.
