The radeon 9600. It's way faster than the fx 5200. Just look on the web at the benchmarks. At the moment, the fx5200 is pretty much a budget card while the radeon 9600 is a somewhat mid-range card.
Unless you were absolutely cash strapped, don't go for the fx 5200. I have on in my Powerbook and while it works, it isn't earth shattering. I wouldn't buy it if it didn't come with the laptop .
I agree with Viro on this one. Definitely the Radeon 9600. I believe the 9600 has more pipelines for data than the GFFX5200.
However, if you are looking into using Linux, my experience has shown that NVIDIA has done the most for support on the x86 side, while ATI is still trying to catch up. I haven't tried the latest proprietary drivers for Linux from ATI, so things might have gotten better for ATI under x86 Linux.
As for Linux on the PPC, it's equal grounding. Neither of these cards support 3D acceleration in Linux/ppc. However, any Radeon up and including the Radeon 9200 series (thanks again, Viro ) does have 3D acceleration support under X.org/XFree86.