I am using cerlon 1.2GHz, 128MB RAM, Asus Motherboard. I have got 40GB HDD is there in my system. Can I add another 80Gb HDD along with the existing one

With a motherboard which supports a 1.2 Ghz processor, the bios probably recognizes/supports 80 GB drives.

What is the model number of the Asus motherboard?

Note that Windows 98 won't recognise a single partition that big. Not sure about Windows ME but I would guess Windows NT would also have a restriction. Splitting the new drive into different partitions (e.g. a D: drive, E: drive and F: drive) would resolve the issue.