20" Cinema display--why not?!?

I'll soon need a new monitor for my G4 DP 450 (gigabit ethernet) with
the (OEM ATI) 2x AGP Rage 128 Pro card. The mid-sized Cinema looks nice
but, according to Apple's specs

<a style='text-decoration: underline;' href="http://www.apple.com/displays/acd20/" target="_blank">http://www.apple.com/displays/acd20/</a>

it requires more video power than I've got.

 > Power Mac G4 with an NVIDIA
 > GeForce2 MX, GeForce3, GeForce4
 > MX, or GeForce4 Titanium graphics
 > card or ATI Radeon 7500, 9000 Pro,
 > or 9700 Pro graphics card

Why? How? The old Rage Pro even has an ADC port.

At first I thought it might be that there wasn't enough VRAM, but
no--even 16MB should be _plenty_ to drive 1680 x 1050 at 24-bit depth.

What gives?

cheers,

Henry<!-- ~MESSAGE_AFTER~ -->