Something I always recommend is to open the PC and look at the mounting
availability space, the current data cable, and the current HD manufacturer
as stated on a label on top of your current hard drive as you may have to
reset the jumpers on it if a Western Digital. Get familiar with your PC's
innards.
If there is no space in the lower innards of the PC for mounting a 2nd
drive, you may have to get a 3.5" mounting for a 5.25" bay for the 2nd
drive. This may require cabling with a DVD/CD device on the same ide ribbon
cable.
And some PC small cases have no allowance for adding any 3.5" wide IDE
devices. In this case, there are two alternatives, a bigger case that most
people can't handle swapping the motherboard etc. to the new case, or adding
an external hard drive.
If you buy a brown box (hard drive only), you will need mounting hardware.
Retail versions sold in stores include this hardware along with written
instructions with pictorials and a floppy or CD.
If PATA (ide), buy a new 80 wire ide ribbon cable along with all else.
Some PC cases are real stinkers for getting to the lower end for mounting a
2nd hard drive. Some are easy.
If two ide devices (hard drive/CD/DVD) are on the same ide ribbon cable, one
has to be master, one has to be slave. This can be determined by the
mounting location on the ide ribbon cable. The one on the end of the cable
must be the master by the ata specification. This can be forced with master
or slave jumper settings for master or slave, or using cable select (csel)
jumper setting if your PC can handle cable select.
Many hard drive manufacturer websites offer alot of insight into hard drive
installation including bios and operating system requirements. Look here
first before deciding the hard drive capacity size that you're getting.
You don't know unless you look, and try.