It is a custom built computer and has had a few things upgraded...
The motherboard is relatively new and we still had the box, but I don't
really know what the other individual components are.
I know that it (or the connection... something anyway) is IDE.. not
EIDE or ATA etc. From what I have gathered EDIE will work with IDE
because it is essentially the same thing. Is that right?

Are there any other compatibility issues I need to know about?

....and about external hard drives, I know my father would still like to
get one. If we replace one of the hard drives within the computer and
bring it up to its maximum capacity, will we be able to use an external
hard drive as well? ...and will *that* be limited to 137GB too?