Buying a hard drive.

I need to buy a hard drive to transfer all my data from my current hard drive so I could format it.

Does it make any difference whether I use an external or internal hard drive? Will I be able to "fit" the internal hard drive inside the desktop without removing the current one and then connect them so I could transfer data from the current one to the new one? Or do I need an external hard drive for that?

Thanks.

 

I've been searching for hard drives all over the net and look what I've found:

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1477509
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1892385

I was wondeing if there is something about these hard drives that I'm ignorant of that makes them so unbelievably cheap? Or are they really low-price and great quality? I'm skeptical of the latter so if anyone can tell me what I'm missing out on, please tell me. Also, what makes these two different? What's the difference between an Ultra ATA-133 interface and a Serial ATA-300?

Thanks A LOT for any help I get on this.