That's not a Cube. The G4 Cube was, well, a cube. That's an early PowerMac G4. Since I don't seen an Apple Display Connector, this is probably a "Sawtooth" model from 1999 or 2000. It would have a 350-500MHZ G4 CPU and come with 128MB or 256MB of PC100 RAM. The last version of OS X it officially supports is 10.4 Tiger, although, if the RAM were upgraded (it supports a maximum of 2GB), a hacked install of 10.5 Leopard would work. It's not really worth much of anything at this point and would be effectively unusable for modern computing tasks.