Ant,

When you speak of a decimal place - a decimal place of what? In regards to temperature we can interpret this to be in degrees C or in regards of resistance we can interpet this as ohms. In regards to voltage - we're talking low voltages 1.xxmV or 1.xxuV?

Anyhow, in answer to your question - some adcs have a differential input which tends to suggest you use a wheatstone bridge as the inputs are normally spec'd to not be too close to the power rails. Differential is used as you can have negative values on a 5V only system - differential in that it measures the DIFFERENCE between the two inputs.

Other adcs are single ended - as in they measure the input voltage between the input and 0V.

So choose your adc and that will determine what configuration you use. If I had to make the decision between using a aduc834 or LTC2410 (as I have both) I would have to read the specs carefully and determine what would best suit my application. All I can suggest you do is choose a solution and run with it. Most likely this will be adequate for your application, if not, you will have learned something along the way and thus have more information on which to make a better judgement. Don't be afraid to make a mistake - I doubt anyone will be injured if you choose wrongly! Don't ever think that just having an engineering degree means that we all know what is the best solution. I've never used a Pt100 before so I have no experience with these, so if I had to build instrumentation I would:
1/ Do some research
2/ choose a solution
3/ build it
4/ test it
5/ if 4 fails goto 2

Most of the fun is figuring out why things don't work!