I'd personally go for the FX5200, even with 64-bit bus. From what I've read, because these have DX9 support, they can run VooDoo emulation through nGlide... which might be something handy for certain late Win9x games that support it.
I'm actually in a similar boat and have a PII 400 MHz PC that I'm thinking to pair either with that (an FX5200... though will see if 64-bit vs. 128-bit makes a difference, as I have both), a Radeon 9200 SE, or a Riva TNT2 (Asus v3800). I've had the 9200 in there before and know that, despite being a weak card, it's not really hampering the performance too much (at least in the games I like to play) as the CPU itself also isn't that capable either. So I think it's a good balance for that system, despite being not-so-era-appropriate. I'm leaning towards the FX5200 for the re-built of that system, simply because I'm curious to see how nGlide will work with a few old games (namely, Need For Speed High Stakes.)
Back to your rig... and depending on the games you want to play, see if you can find a GeForce 4 MX 440 or 460 instead. Those would be a good match for that PIII and most DX7 and older games. In fact, these two cards have better DX7 performance than the FX5200.