I almost bought a GeForce FX 5500 card this weekend, but held off till I speak to someone at Castle about it. I'd really like a GeForce FX (which are the last cards with a plain PCI interface, I believe), but a GeForce 4 would be good also.
The 3D driver already includes support for these cards and should work fine, although you do of course need an updated nvidia module to bring up the card under RISC OS.