No they'll only have to code it for each architecture. Nowadays both Nvidia and Ati use some form of a scalar architecture, meaning that the lower performance cards are derived from the higher performing cards. So if we say that the game devs needs to support cards 2 generations back, that'd mean 6 profiles at the most. It does sound like a lot but is not unrealistic at all. And unlike on the consoles, you'd still have control over all the graphical settings and will be able to scale them up or down individually according to your needs. Still I'm speculating now and we won't know for certain until somebody tries it.