I'd say look for an FX5500 instead of FX5200. This flashing experiment started
here, and followed up
here. The ROM can be found on the same Strangedogs forum, just look inside.
The reasons you want to pick FX5500 over FX5200:
Cube-fitting FX5200s are mostly 64bit, which runs slower than Radeon 9000. 128bit version of FX5200s are usually too tall for the Cube, requiring a VRM move. If you can spare the time and money, (about 1-2 hours of your time, and shipping for the extension cable) then the ultra FX5200 would be the one to get. If you can't, FX5500, being 128bit, is almost as good as FX5200 Ultra.
Radeon 9600 is not an option at this time. Ruby of Strangedogs is supposed to be working on better nVidia cards (6000 series) and Cube users are counting on her and the card-flashing extraodinaire Arti.
And how does Doom 3 run on a heavily suped-up Cube? Not too shabby actually. You have most of the eyecandies, but just don't expect perfection. 800x600 gameplay should be quite adequate. However the biggest bottleneck appears to be the bus speed. At 100MHz the Cube is very sluggish, with framerate sometimes dropping to single digits. This you find out very early on as you follow your bot down the underground passage*. Don't get hot and bothered. Just remind yourself that the latest iBook performs just as bad, if not slightly worse. And that baby has a Radeon 9600 on it. (32 megs or not, it still is a Radeon 9600, a 5200 Ultra-beater.)
*I serious doubt a Radeon 9800 will solve this, seeing as this is most likely a CPU/bus speed-related issue.