Best pair would be the Bosch PS32 (the brushless drill/driver) and the PS42 (brushless Impact driver). Aside from the drawback of being longer in the schnozz and slower to change bits, the drill/driver can do everything the driver can do just as well, plus it's a better drill. In order of getting them individually, hands down the drill/driver is first, then the impact, then the screwdriver.
Full disclosure: I've been in the Bosch 12v platform since it came out here in the US. I currently have the PS32 (brushless), 2 drivers (PS 20 & PS21) and the impact driver (PS41 IIRC, could be a PS40 though). I have the 2 drivers because when they initially came out with a 2 tool kit, it was the screwdriver and drill/driver, which I upgraded to the Brushless version last fall. The second one was a freebie/rebate beast that I got along with the impact driver. It is very convenient to have a drill/driver and a separate driving tool or two available, so I wouldn't worry too much about redundancy if you end up with the screwdriver and impact.