I agree with Kai. If want to calibrate every thermistor you will get your stated accuracy. A PT100 sensor whilst more expensive is much more accurate. You could use something like Analog Devices ADuC824 to directly interface with a PT100. I think they have an app note regarding this and it has an 8052 core! The eval board is only $75US. You could use some analog parts (op-amps etc) to amplify the resistance change of the PT100 but there's lots of hidden issues here. Failing that, buy an interface that copes with the interfacing details.

There's plenty written about temp measurement on the web.