Nice work. Was the video card a flashed PC version by any chance?
Most definitely. AFAIK, there never was a Mac with a GeForce 6200 AGP card. Not even sure where the 6200 Mac ROM came from (was there an iMac with a similar nVidia chip?), but these cards are all over eBay. Other than the hard to find Radeon 9700's and the rare & expensive black PCB Radeon 9800's, I think the 6200 is the 'best' available card that will fit in a Cube.
I have a standard Mac Edition Radeon 9800 in the G4 tower that I may try to squeeze in to the Cube if/when I get a PowerLogix Clear Case, but it will most assuredly have to be externally powered. But I suspect the card is simply too big regardless of what enclosure the Cube goes in to.