Erik, the poor guy is having enough troubles understanding the fundamentals - lets not complicate it too much at this stage!

Ant, I suggest you do some trolling at Analog Devices, Crystal, Linear tech ... as they have a wealth of info on interfacing to Pt100 devices. Expect to have a steep learning curve as what you're trying to do is not quite as simple as you think. You're trying to detect very small resistance changes accurately. This translates into small currents and voltages where every wire connection can be a thermocouple adding to your errors. Also, at this low level small bias currents can have a major effect on your readings. One project I did, I found even the leakage current though a 1N4148 diode could introduce significant errors so I had to choose a particular low leakage diode to minimise these effects. Also, at this level, the resistors have a temperature co-efficient that can also add to your errors. So, whilst the Pt100 is a very accurate sensor, to achieve its accuracy at the measurement end is not so easy. I dare say you've priced the instrumentation for these devices - you pay for the accuracy for good reason! This is not to discourage you - by all means give it your best shot, but be aware of the potential problems! Just breathing on the circuit could give you erros!