Originally Posted by DoomUK
Or does OpenGL not work that way; is it merely a question of raw performance?
I've not played D3 on a pre-DX 9.0 card, but my guess is that effects like "Heat Haze" won't be rendered. Possibly normal mapping also won't look as good. OpenGL doesn't have the DX categorisations, but there are ARB reference card capabilities - things like SM 3.0 are standards that must be supported in hardware under DirectX, and the hardware limitations don't change, no matter what API you're using. In other words - even though OpenGL doesn't have an official SM 2.0 spec, a card that supports exactly the requirements of SM 2.0 will still have those limitations on shaders under OGL.
In other words, although QIV may support older cards, the most advanced shaders won't be rendered on them. As for being DX 9.0c compliant, all that really means is that the card words with recent versions of the drivers - The GeForce 3 is pre DX 9 in feature set, and everything on that list 'cept for the NV4x/G7x/R5xx cards is pre "DX 9.0c".
EDIT: Although I note that the R5xx cards actually aren't on that list, outside of making jokes, of course they're going to run the game.
EDIT 2: And as a minor Corrective to your post, the GeForce FX/Radeon 9xxx [except for the 9200] cards ARE DX 9.0 compatible (a & b respectively), it's the previous generation (GeForce 4/Radeon 8) that aren't.