assuming that the utility that dusty provided the link to, delivered +ve results, there's really no reason to check if the drive is empty or not. during installation, XP will give you another chance to wipe the drive clean.(a good advice is not to use the quick format option) after that, no matter what, the drive will be empty. mind you, you'll have to delete the partitions too, and repartition the disk according to your wish, which can all be done very, very easily.
but wait, you'll still have to observe the functioning of the drive while it is still in pc#2. check if you can transfer some files to and from the drive in question to pc#2's original drive (assuming it isn't disconnected.) it is not very smart to totally rely on a utility provided by a manufacturer, although it may be necessary. hard drives can be tricky some times, heavy usage tends to make them marginally slow compared to the new ones, since they are the only functional moving components of your computer that your PC can't do without.
anyway, if your HDD passes this test, it is ready to be put back into PC#1.
as for your second question, to set up multiple HDDs on your system, you need to make sure that you have 2 dual channel IDE cables installed on your system(which is usually the case). IDE cables connect HDDs to your motherboard. in case you don't know, 2 cables with 2 channel each gives you 4 channels to play with. so you can connect max 4 HDDs on your system. but at least one of those channels will be taken up by your DVD burner that leaves you with 3 remaining.
when you look at an IDE cable, you'll find 3 connector pins, 2 at the ends, 1 in the middle but a bit close to one of the other two. the pin that is isolated is to be connected to the motherboard, the remaining 2 can be connected to the IDE devices (HDD, CD, DVD etc.).
now, the device that you'll connect to the other end of this cable will be a master drive by default, and the one you'll connect (if any) to the pin remaining will be slave.
besides this, you'll have to set the jumpers on each device before connecting them to the cables. and no, you won't need any silly pics to go about this. just look at the back of your HDD, where you connect the cable, there will be a set of 8 metal pins in 2 sets of 4s. there will be a tiny plastic/metal connector on top of 2 of the pins in there, that is your jumper. a Jumper is a device used to short 2 pins, the manner in which the pins are shorted, determines the functioning of the device in question. every IDE device has it's jumper setting. you can place this tiny connector in such a way that determines the behaviour of your HDD. and the manner in which you should connect this jumper is completely specified on the back of your hard disk in a figure. it should specify which 2 pins you should connect the jumper across in order to connect your HDD in master or slave specification. Follow my advice and connect the jumper in the 'cable select' position which is a sort of default specification to get rid of conflicts. and don't be alarmed if you find more than one jumpers in there, which is likely coz u have a samsung drive. Just follow the instructions that are ON THE HARD DRIVE and not anywhere else.
OK, so connect one HDD on the master end of your IDE cable with jumpers set as master/cable select, connect the second HDD on the slave end of the IDE cable with jumpers set as slave/cable select, connect the power and you're good to go!!
alternately you can also connect one of you HDDs on one IDE cable as master/slave (with correct jumper settings of course) and the other HDD on the other IDE cable, independent of the other HDD, as master/slave as you wish.
connect your DVD drive on any of the remaining positions on the IDE cables, with it's own jumpers set properly.
as for your 3rd question, there's no real need to fromat the 3rd HDD, but it would be wise to format it just to resolve conflicts if there is an Operating System installed in it. your computer won't know which one to boot from it haven't installed multiple OS on your system, and in any case, if there is a crash, the data on your second drive should remain in dependant on the first drive, unless of course, there is a fatal virus infestation on your system.
GOOD LUCK!!!