[caravela]

For 512mb boards which is likely to be the most optimal setup, a GPU/ARM split of 128/384 or 256/256?

that depends on what you use your pi for, if you just run Xbmc 256/256 or 128/384 should be fine. if you are gonna have services running like uPnP, webserver, transmission, samba i would recommend the 128/384. Again just depend how much stuff you have running, more stuff more ram for the system

EDIT:just tried 128, 192 and 256 by bare observation i couldn't see any difference