The exact curve is given in DIN IEC 751:

For 0°C...+850°C:
R = 100 x (1 + 3.90802 x 10^-3 x T - 0.580195 x 10^-6 x T^2)

For -200°C...0°C:
R = 100 x {1 + 3.90802 x 10^-3 x T - 0.580195 x 10^-6 x T^2 - 4.27350 x 10^-12 x (T - 100) x T^3}

T is temperature in °C and R is ohmic resistance of PT100 element.

You can see from the formulas, that the curve isn't linear! Either you do the correction by software or you can also do it with your front end analog electronics, by the methode of positive feedback. This will theoretically allow a maximum precision of +-0.8K in the range of -200°C...+800°C, +-0.04K at -150°C...+300°C and +-0.01K at -50°C...+200°C.
You can also use chips, which already provide this positive feedback, internally, like XTR103 for instance.

Kai