LOL @ the idea ACube or Hyperion is getting a kickback out of ATI.
The number of cards they would be selling is peanuts. Not even a blip on the radar.
The ATI 9250 is a card from 2002. While useful on the Amiga platform both for PPC/NG folks and expanded A4000/A1200 users alike, they are mostly being used as doorstops and paperweights by the rest of the world. Surplus DirectX 8.1 cards that no one else has a use for. $10 items off ebay.
The hardware will support whatever card you throw in there, it's just a PCI slot. For whatever reason, the OS supports only ATI cards. I'd have to think that the developers of the drivers chose this line of cards because they were cheap and plentiful, and writing drivers for them was possibly easier than it was for the nVidia line.
If you want better gfx options you'll have to look at the 460 or the X1000.
I had a 9250 in my ep and didn't notice it offering anything better/more than the onboard gfx chip did, so I removed it, but you don't have that option with the Flex boards.