The Spirica Video Card mount kit for Radeon 9000 supports Radeon 9000 64MB/128MB Mac Edition. You cannot install the Radeon 9000 in the Cube because the card is too wide and interferes with the VRM. You need to relocate the the VRM, which is possible with a PowerCube for example.
This kit also supports nVIDIA Geforce 4 MX 32MB ADC/DVI, 64MB ADC/VGA and 64MB ADC/DVI. But you need to cut the edge of these cards.

Remove the origial bracket first. You need to remove 6 screws in total. Then put back the 4 screws on the each side of the video connectors.

The mount kit consists of L-shaped angle, each with a single screw and washer. The angle has thread holes, so you can use the original screws.

The ADC lock interferes with the Cube Chassis. You need to either remove the lock or grind the chassis so as to make space for the ADC lock. It's your choice.

That's it! You can now install the video card firmly. This will prevent your Cube from unstable operation or serious trouble, such as a logic board failure.