You would not get much better resolution, if you 'd carefully scale input to 0-5 range using OPA, compare to simple R - RTD voltage divider.
Other options, use external ADC, 16 - 24 bits, or you may dig for Atmel app. note explaining oversampling technics AVR121.
For update rate 3 Hz , which is (IMHO) o'k with temperature measurements, they give this numbers:
Voltage Oversa- Right Ideal Ideal voltage Maximum
resolution mpled shifted decimated representation Bandwidth
~75 uV 4096X 6X 32413 2.4729V ~3Hz (1)