Its most probably the buffer size you are talking about. Just to put it in layman's terms, the bigger it is the better =) And yes they will work in your computer.
Also it will be good to take note how you connect them up.
I would suggest connecting the new one as master and only one for one IDE slot and on the other connect the 20gb as slave and cdrom/dvd drive as the master on the other ide slot. If you don't understand what I'm saying then its better to do some research on the net about master and slave settings

Nothing much to it actually. Very simple.=)