basically there are two types of configurations
you can just add it which will mean that it appears as harddrive D: on "my computer" you will need to format the old hdd to remove the xp filesystem as you will want to use the full space of the hdd
you can raid which is where the disks become parry to special elements in programming but when you make a raid array all harddrives within that array get formatted and so lose all data. This isn't a problem you can back-up your data and and just re-install the Operating system (windows vista) after building the array I install a new operating system about one a month on my laptop. Raid arrays have their advantages the two basic ones are Raid) which is where both drives are added together to form one virtual drive this virtual drive has better performance as it can access the data quicker as two minds are better then one
. Raid 1 is where the data is mirrored this means an identical copy is made of one of the hdd's so if the hdd ever fails you have no issues as you have the backup hdd.
raid gives you a performance boost, standard doesn't but you don't have to reinstall the OS we can help you with both and you can ask as many questions as you like until your comfortable with what your doing. We are all here to help each other
make your choice then we will help you with the details with each