I installed ubuntu on my machine with only 80 GB of memory anticipating that I will remove it later but now I want to keep it forever (until I am frustrated with linux). I have 500 GB in my machine and now I want to use that raw 420 GB of space. How I can I do that ?

with "space/memory" I am referring to secondary memory not Ram.

Here is output of :

sudo fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes 255 heads, 63
sectors/track, 60801 cylinders, total 976773168 sectors Units =
sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512
bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000dcb77

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 136718335 68358144 83 Linux