If all my data is in UTM NAD83 Zone 10 and I get new data in GCS WGS 1984 do I have to reproject the WGS data to UTM NAD83 Zone 10 for it to align correctly or can I just bring it into an mxd that is already in UTM NAD83 Zone 10. If I need to reproject it using ArcCatalog how do I do this?
If you are planning to perform spatial analysis using your data, I suggest you to reproject all your data into one projection, either using ArcGIS or FME. Sometimes the reprojections does not work perfectly on the first try, you may need to try a couple times to get it right.
I know it is dated in May but if you don't reproject it to UTM NAD 83, then your ArcMap will run very slowly !! so if you reproject then it will run smoothly and I agree with Christina as well.