you didnt mention its gonna be an external , usally any external should fit considering nearly all have usb+firewire connections and all pcs in the last 7yrs or so have usb so i thought your asking cuz its gonna be an internal one , external hard drives are good for storage/playing audio+video/some light video editing/running low profile games but are defintly not idle for running an operating system from , cuz they are slower and not every motherboard allow booting from usb storage devices , however external hard drives are not special hard drives in any way , they are actually standard hard drives in an external enclosure.. so in thoery you could open it and take out the hd and connect it internal.. (as long as its in the right inches,see some externals are 2.5 and those drives wont fit in a desktop pc,and as long as you got an available ide connection..) , anyway if your gonna get an external theres several things you should know
a) all one peice external hard drives have only 1yr warranty,in rare cases 2yrs , that is why its best to buy the hard drive and the ext
ernal enclosure in seperate , youll get more warranty at least 3yrs or 5yrs if youll buy a seagate , i highly recommend avoiding maxtor/hitachi as they are the worst , youll have to buy an ide hard drive along with that external enclosure , sata drives are not idle for externaling so to speak , btw did you know that every one-piece external hard drive have an ide drive inside?..
b) when buying in seperate , you can pick between 2.5" to 3.5" hard drives , basicly the diffrences are , first of obviously a 2.5"+external enclosure will be way smaller+far less heavy , also 2.5 external enclosures are bus powered means no need for an ac adapter to plug to the wall socket.. , on 3.5 enclosures ac adapter is always needed , also considering 2.5 hard drives are made mainly for laptop use they should survive in higher temperatures.., the downside is that 2.5 hard drives are much more expensive then 3.5+lower in capacity
c) materials , there are two for external enclosures , plastic & alumium , its best to avoid plastic as its bad for heat disposition , aluminum is the better choice by far
d) some external enclosures have a fan , on some its 40mm and on others its 80mm , theres no sceintific proof but its believed a hard drive will live longer when cooled , so i highly recommend getting one with a fan perferably 80mm , 2.5 external enclosures dont have a fan or at least i havnt seen one with a fan but thats alright from a reason mentioned in b)
edit btw firewire is faster then usb by about 10%