Os x ?

Hi I have a iMac G5 2.16 intel core 2 duo Ram - 1Gb 667 MHz DDR2 SDRam Mac os x version 10.6.8 I am new to macs I want to format my Mac and start fresh I have no instulation disc I am wanting to buy new ones and not sure on what OS X to buy regards kon

This isn't a G5, it an Intel.
Probably have an empty RAM slot that can take an extra 2 Gig and that will give to enough to go to Lion.
If your internet speed is reasonable... buy Lion at the App Store and install. Boot off the Recovery Partition and then run Disk Utility, Erase the disk and Re-Install.

Else buy a Lion Stick.

Either Way you'll need to backup your Apps and migrate them back on as the Lion doesn't include iLife, or you can just buy them from the App Store after installation.