PhysX works Nvidia cards all the way back to the 8xxx cards, while TrueAudio is for new cards and Mantle is for all CGN cards(7xxx and new i think.).
I am not sure what you want for an answer here, but user buy what they need(lets say you want a card with older connections like dual vga or s-video for and old htpc setup. This has to be getting rare now) or can afford(getting a 560ti is faster than a 650ti, it consumes more power, but the card may be cheaper to buy[even more so second hand]).