White line - normal operating range.
Yellow line - maximum engine speed. Do not keep in yellow continuously.
Redline - maximum engine speed. Should not be exceeded in any circumstance by a stock engine.

These stock operating limits are approximate due to the errors inherent in tachometers. If you get a tacho calibrated you will notice it can be correct at one speed (say 8000) but be 400 rpm out at 4000 rpm.

To raise stock engine speed limits requires engine balancing, component lightening and valve spring/retainer/rocker upgrading.

To actually get the engine to be able to use the increased engine speed range (as B120 Dat mentioned) a camshaft change, ignition upgrade and a suitable fuel supply is needed.

I have also had the pleasure of a near stock 1200 that would run to 6600 in top gear. It was getting valve bounce at this speed : )

The car was great fun, especially commutting from Queenstown to Burnie in Tasmania via the Tyndals road to Tullah. If you ever get a chance to do this drive in a 1200, it is a must. In fact the whole of Tassie is 1200 country