Don't buy an external one. Besides what Jedi said, they have a habbit of cracking fast.
If your current hdd is also about 1-1.5tb you should look into setting them up in RAID-0 for the extra speed. I'm too lazy to look up if your motherboard has hardware support, if it doesn't, software raid should be ok. Not as good, but still a nice speed boost.**
Your options are, basically:
Western Digital for better power consumption (green versions)
Seagate for better (and by better I mean so small you would never notice) speeds
..and maybe a HITACHI with which I have no experience so dunno anything about.
My personal suggestion is to go with WDC. They make badass drives: quiet and reliable. (
emag linky)
**if you're gonna go for raid-0 there's a bit more you should look into when choosing a drive, like compatible speeds so you don't have one slowing down the system. Also, how old is your current one and when did you check it's health last time?