I thought about upgrading my Cube graphics card to the ATI Radeon before it disappeared from Cube availability, until...
I realized that the Radeon has an onboard fan. One of my favorite things about the Cube is it's quiet operation, and the fan would just ruin my day.
FYI: Apple used to have a disclaimer at the Apple Store that (parapharsing) "due to the Radeon's onboard fan, Radeon Cube's would not operate 'silently'."
I don't know if this is an issue for me, but I thought I'd point it out.
Have fun!