Cheapest option would be to pick up a PC 6800GT for < $40 and flash it yourself. Go only for the exact cards though that Mac Elite offer ROMs for. I've a Leadtek 6800GT, supposedly the A400 model, but with a cheaper heatsink, that plain refuses to work with any ROMs tested (might try the other ROMs just to see though).
The PC flashed 7800GS is the best card, but again be wary as you can't flash cards over a certain ROM revision as they essentially use the same chipset as a 7900GT, which isn't supported.
JB