Here's one for ~$600, it gives an okay performance.
http://configure.us.dell.com/dellsto...-inspiron-1545
Change the CPU to the T4300, RAM to 3GB(3GB is a good bang for the buck), GPU to 4330, and batteries to 6 cell. You can also change the OS to W7 if you want. Not sure if Dell ships to Singapore though.
Edit: It weighs 5.6Lb, or 2.7kg.