If your machine only has one drive currently connected you could most likely just add another drive to the same cable the other one is connected to making sure to get the master/slave or cable connect settings correct.
You should check one important detail. Up until about 10 years ago, most computer mobos and their built-in IDE controllers could NOT use any HDD over 137 GB (as measured by drive makers - Windows calls this 128 GB). So before you buy a larger IDE HDD, check the details with the mobo maker's website. You need to have what is called "support for 48-bit LBA". Note that "LBA Support" or "28-bit LBA Support" or "Support for large hard drives" is not good enough - it has to specify 48-bit LBA.
If you do not have this in your machine, many of that age could get it by downloading and "burning" an updated BIOS file from the mobo maker. So check that possibility.
NOTE: this problem does NOT apply to SATA systems. So, IF you have SATA ports built into your mobo and you're simply not using them yet, you CAN buy a SATA HDD and plug it in there. ALL SATA controllers do have this 48-bit LBA Support feature. Alternatively, if you buy and install in a PCIe slot a SATA controller card, you also can use a SATA drive of any size. The possible problem I outlined is just when you try to install an IDE HDD over 137 GB in an older machine.
One other thing to take care of before trying to install a large HDD. The 48-bit LBA Support also is required in your OS. The original version of Win XP did NOT have this. It was added in Service Pack 1 and maintained in all subsequent Win's - XP with any SP, Vista, and Win 7. So If you are using only the original Win XP, upgrade it to SP3 before doing anything else.