Why do you want to upgrade rom.Your phone should be running stock ROM.No need to update anything. Connect it to Wi-Fi,go to Settings>About Phone>Software Information>System updates/Software updates/Check now.If it prompts you an update,then download and install it.Thats it.
N Sense 3.0 is made for Dual-Core phones.
Sense 3.5 is 'Lite' and its made for Single-Core phones with high optimizations.