That's a flashed PC card, which won't be any more compatible than the 9800s. Less so actually. I'd stay away from it. It's actually not that hard to flash your own Radeon 9700/9800, and the Strangedogs forum has a nice tutorial.
Here's a
link to a reseller offering a "real" 9700 Pro Mac Edition (out of stock, but they can probably get you one). Notice that it is a green board with a DVI
and ADC port. Many of the sellers on ebay sell flashed PC cards. This isn't necessarily a bad thing, but they often neglect to admit they aren't selling a "real" Mac card, and some flashed cards do have issues in certain Macs. A flashed PC card would be less compatible than the 9800 Pro Mac Edition you were originally considering.
Barefeats did a review comparing the 9700 Pro to a number of other contemporary cards
here and
here. Notice that performance in a G4 is nearly identical between the 9700 and 9800.
In short, I would avoid flashed cards unless you DIY (there are members of the Strangedogs forum who will flash a card for you generally they offer good prices and are up front about which cards work in which Macs. If you are concerned primarily with compatibility, get a 9700 Pro Mac Edition. It won't be cheap, but it is the "correct" card for your computer, and has very good performance. Alternately, get a 9800 Pro. It will run a little hotter and there is a small risk of incompatibility, but many MDD owners are using them with success.