I'd keep my 9800 Pro, but it's gone to artifact hell despite sitting unused in storage for a long while. What a shame.
Now, while they're generally outclassed for a number of reasons, there is one use for which they might be handy: they're amongst the easiest cards to flash for Power Mac G4 use, no need to track down a Mac Edition. Thing is, so are the more powerful X800 cards (and FireGL X3 deriatives) from the next generation, so if you don't already have a 9x00 card, you're better off springing for one of those.
That's exactly why I broke the 9800 Pro out of storage to begin with, until a quick POST revealed the sorry state it was in.
As for the GeForce FX line, I saw no value in it initially, but they're a bit more well-regarded in this community when it was discovered that they still do 8-bit paletted textures and fog table, as well as theoretically supporting proper shadows in Splinter Cell (but that game's so finicky of a port job that just having a driver version that's too new will make them render incorrectly, hence the push for using a GeForce 4 instead). GeForce 6 Series apparently can't do any of that, neither can Radeon 9x00.
Too bad my FX 5950 Ultra has what looks like a tantalum cap busted out of place, one tab snapped right off where it meets the packaging, and I can't even read the values on it. I need to fix that part up so I can test it. Even then, I wouldn't dare replace my 6800 Ultra with that sort of downgrade on my P4EE box; SM2.0 performance that doesn't suck and SM3.0 support is important to have for how hard I push the XP side of gaming on that thing, Doom 3 and Far Cry included, and heck, I should test F.E.A.R. on it as well.