OS X used composited desktop with NO acceleration AT ALL for a long time.
They didn't have acceleration until 'Quartz Extreme' came out. You know what the hardware requirements were for GPU driven compositing?
* AMD (ATI) Radeon, AGP-based, 16MB VRAM minimum, or better
* NVIDIA GeForce2 MX, 16MB VRAM minimum, or better
OS X 10.0 came out over EIGHT YEARS AGO with a software-driven composited desktop. This thing ran fine on PowerMAC G3 with 266mhz processors. I know, because I had to set them up for a part-time job I had at the time. (as long as you had enough RAM)