The answer has to do with the interface to the external drive. Assuming you have SATA drive support and an ESATA port to your external drive, yes it should work.
However if you have a drive bay available inside your current machine, the installation is much simpler.
SATA rings a bell, so I assume I have it. I'm not sure I have a spare drive bay though. At the moment I have two 750gb hard drives in RAID, so I might not have any space. I'll have to check my computer's innards when I get home next week. Is it normal to have three drive bays?
I agree that in internal drive is simpler. Do not buy an external SSD that has USB connections. (Unless USB 3.0) All the performance would go to waste when connected to a slow USB 2.0 port.
Ok, that answers one of my follow-up questions about a USB connection
And in that case, I'll focus on the possibility of an internal SSD. If I have space I'll ask some more questions about recommended size and manufacturers. If I don't have space for another drive, then I'll leave it until I buy a new computer and stick to my hard drives.
Thanks for the replies ^_^