if this is just a MAMEputer .... 30GB is overkill
... a 9GB HD would be plenty for the OS ... finding anything small is getting kind of difficult these days

excellent idea though ... keeping the OS and ROMS on separate HDs. I'm even sure 20G could hold all your ROMS and then some ... I could be wrong though ... not sure about what emulators your running.
and even better idea would be to make the second HD with the ROMS part of a RAID 1 array

btw ... Partition Magic is a must ... even after you've made your partition size decision ... if you would happen to make a partition to small and start to run out of space ... you can go in with PM and adjust the partition sizes again if you have available space on another partition to spare. Excellent software to have in your arsenel