What resolution are you looking for? By my calculation, if you make a potential divider using a PT1000 and a 1K resistor, and connect that directly to an Arduino analog input pin, you can measure the temperature with a resolution of 1 deg C. If you want higher resolution, you'll need an op-amp to amplify the voltage change.
Using a constant current source is all very well, but then both the current source and the ADC reference have to be stable (or the current source has to be controlled by the ADC reference). The potential divider (whether amplified or not) driven from the same voltage as is used for the ADC reference has the advantage that it is not very sensitive to changes in the ADC reference.