I found this link for creating custom memory partition on sd card-
HTC Salsa C510e Rooting - Englebert Life
At the end of the tutorial it says:
"basically the htcfs is mounting the /data from my sdcard on every reboot before all the applications start up. There is a step to copy all existing /data to the sd-ext and that is the trick to make the phone thinks that is using the internal memory and we have no longer facing insufficient disk space"
Will this work once i have my phone rooted?