Hi. I wouldn't jump to any conclusions just yet. My oldest computer I've had since like 1998 makes clicking sounds from the hard drive too. It's been making that sound for like 3 years now and it still functions. I don't store anything important on that drive. I mainly use that computer for downloading songs and stuff like that. Anyways, If anything important is on that drive, I suggest you back it up now. As for what a good drive to buy is, I would go with a SATA(Serial ATA) over a PATA(parallel ATA) drive. The difference is:
Thinner cabling due to SATA drive sending data serially instead of in parallel which requires far less wires.
The cable length is more than twice the length(39.4 inches(SATA) opposed to 18 inches IDE(PATA).
SATA is hotswappable(you don't have to power down the pc)
SATA's serial data rate is like 30 times faster than PATA.
If your Mobo doesnt have SATA controller sockets, you can add one via a PCI card.
I found this link for a 100 gig SATA drive. Not bad price.
http://www.techs4you.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=MX076L100M0
So I hope this helps. Maybe when you get your new drive you can use the old one as a backup or second drive.
Oh yeah, I almost forgot, of course internal.