Great find, and you can't beat the price! I love the whole series of ADC monitors(I have at least one example of every one, as well as two 17" and two 23"). They are a perfect match for a G4 tower or Cube. I love the how "minimalist" the set up is with a G4 tower-plug the tower into the wall, the monitor into the tower, the keyboard into the monitor, and the mouse into the keyboard.
Be aware that a Cube with a stock Rage 128(the most common video card found in these) can't drive a 20" Cinema. You need to upgrade to a GEForce 2MX or Radeon 7500(at a minimum) to drive the 20" monitor, and even at that you're making these cards push a LOT of pixels. They also have issues in that both run hotter than the stock card, although the 7500 runs a bit cooler than the 2MX. I had the heatsink on a 2MX raise a blister on my finger when I didn't give it enough time to cool down before pulling the "core". The 2MX was a BTO for the Cube, but the Cube-specific version had a huge aluminum heatsink that covered a large portion of the card. The more common tower version has a much smaller heatsink. The advantage to this card is that you can swap the(Cube specific) faceplate from the Rage 128 onto it(the Cube plate won't fit a 7500). You should install a base fan with either of these cards(or really even in a 100% stock Cube) although a base fan doesn't give a lot of air circulation over the video card.
A high end Quicksilver(or even a not-so-high end one) would make a great OS 9 gaming computer. The dual 1ghz would be the best, although the second processor won't generally give any advantage under OS 9. The advantage of the Digital Audio, Quicksilver, and MDD for gaming is that they give you a 4x AGP slot as compared to the 2x on earlier computers.
Many folks-with good reason-consider the single 1.25 MDD to be the king of OS 9 machines, and I do have to admit that there's good reason for this even though I have a bit of an aversion to MDDs(the PSUs are known to be flaky, and I have a closet full of dead ones bearing that out).
If you go with a G4 tower, you also have access to a much wider range of GPU options. The best OS 9 card you can get is the GEForce 4Ti, although these are tough to find and can be pricey. I've also had some reliability issues with them. I've bought four, one of which was DOA and the other of which lasted about a month. My other two continue to work fine, though.
Both the Radeon 9000(stock MDD card) and GEForce 4MX(BTO on Quicksilvers) are good budget choices and generally give very good performance. They are also both fairly common and inexpensive(I've bought both for under $10 each). Just be aware that if you are installing an R9000 in anything but an MDD, you will probably either need to use an MDD OS 9 install or track down the appropriate ATI OS 9 drivers. Neither is a huge deal-I think it took me just a couple of minutes on Google to find the drivers a while back when I was hunting for one.
The great thing about all of these cards, of course, is that they all have an ADC port so you don't have to worry about the A1006.
