what external h/d do i need?

hello i have got a minimac 80gb running with leopard 10.5.6 and someone advised me to get an external h/d so i can run time machine,i tried backing up my games files to to a dvd-rw but it did not like the dvd.
can someone please tell me what size external h/d do i need and suggest a fairley inexpensive model as i only want to backup my music and games ?
thank you.

thank you i have found a 120gb drive for £45 from smalldrives.com
i think that should be enough storage for time machine