Basically, if that dual 867 MHz machine was 'naturally' a dual 867 MHz machine, (which means it is a Mirror Drive Door from January 2002,) then you have no limitations at all.
If it is an older machine that was upgraded to dual 867 MHz, then you could have a limitation that the onboard hard drive controller will only recognize up to 128 GB drives. As long as you use a 120 GB or smaller drive, you have no 'partition' limitations.
If you happen to have some oddball beige G3 or original iMac with some unheard of dual 867 MHz upgrade, then you would have to make a less-than-8 GB partition for OS X to reside in.
Finally, if you're going to put the two OSes on the same partition, then you can go ahead and use the whole drive. If you're going to have separate partitions, then pretty much just decide how much space you want for each. Mac OS 9 needs about 200 MB, although that is for an extremely stripped-down install. Mac OS X 10.4 (not "X 4 or X.4") needs about 2 GB, although, again, that is for an extremely stripped-down install. On an 80 GB drive, 10/70 or even 5/75 would probably work fine, unless you know you're going to need more room on the Mac OS 9 side.