They may emulate these DX9 calls in software, like DX7 cards (GF4 MX) do
with DX8 calls. Looks just the same, fair bit slower because it's not all
hardware accelerated anymore.

DX8 and DX9 pixel shaders aren't software emulated at all, so you won't see
these on older hardware. Depending on how the game's designed it may look
vastly different or not much different at all. With HL2 it will look a bit
different. Doom3 will look almost the same, no pixel shaders means gradients
won't be as smooth but it will have the same basic appearance. HL2's effects
can't all be replicated in the same manner using cruder methods, so those
effects will be missing entirely from the game.