Your cube is very similar spec'ed to my G4 450 Tower with 768MB.
I find Metro good in Jaguar. 10.2.6.
I find it more stable than 9 although I think 9 is quicker when you start pushing the machine to it's limits with multiple tracks or effects.....but OSX is my preferred system to use metro in.
The only big limit is having everything on one hard disk.
If you were to connect a firewire drive to the cube and spit your audio tracks to it, things would be greatly improved.
A cube with Metro SE should be fine and even better in 10.3
scoot.