I don't have the figures, but I would guess that 500 MB and 256 MB RAM covers at least 90 percent of computer users in the developed world. The 3D graphics card would be the problem, since these are not included as standard in budget PCs. I don't have one, though I bought one for the kids' computer so they could play Monkey Island 4 and the Sims. But I am sure that 100 percent of hardcore gamers have 3D cards. I suppose it depends on whether your game is aimed at the casual market.
I would also add that my main PC (the one without a dedicated graphics card) does run Direct X 9, but the last indie game I tried that said "Direct X 9" didn't play in one release, and did in the other. As a non-hardcore gamer I avoid games that require graphics cards, though I appreciate that nearly every big name game needs them.