Your new drive is at least a 7200 rpm with cache and the 20 max probable4800 rpm and probably less. the 120 G. I would use new one as master because of speed.
I would also take the opportunity to partition your drive. If something goes wrong with XP and is not repairable, then you won't lose all apps and data that are not backed up.
I have a 10 GB partition for XP, my apps are on D and data is on E. I also dual boot and use
this freeware script to make images and incremental backups. You do have to install .net framework.
augie