PersianMafia, sorry about my previous posting. I have to reword my response because upon re-reading the question, it seems more than your usual 'Which HDD is better?' type of post.
I'm not an expert at all in HDD storage but i'll answer some of the points raised in your question.
PATA hard drives
The size of your new HDD should be based on factors like how much data you currently have, applications/uses for your PC (such as video or graphics editing, home pc server, etc.) and how long you plan on keeping your PC.
The newest ATA standard (Serial ATA) offers the fatest data transmission rates. However, you shouldn't be too quick to buy one unless your MB supports SATA HDDs. Many users w/ older systems who have bought a new SATA HDD use a PATA -> SATA cable converter... but IMHO, that's it's really not worth it, unless you plan on upgrading your MB in the near future. Cuz the increment you'll notice in the overall speed is better but you definitely won't get the maximum throughput speed of a SATA.
External/Internal: It's totally upto you. As long as you have another IDE connector available in your PC, you can install an internal HDD. The primary advantage of an external HDD is the portability factor and this is particular good for people with multiple PCs they'd like to backup.
Generally speaking external devices (such as HDDs and CD burners) are slower in speed compared to an internal but you can get a decent external unit with Firewire connection that will give you a definite boost in speed. (Firewire is still faster than USB 2.0).
Basically it depends on your budget and your personal preference. Personally, speaking I like Maxtor and Western Dig.
