I was browsing on Ebay and came across some Ralco RZ branded lighteweight underdrive pulley's, I was wondering if anyone had bought these before and if they were any good/ is it a actually a useful upgrade?

 

Go with an oem style dampened one

 

Any examples? Or is that simply making the current one lighter?

I know these are probably stupid questions, but I really don't know a great deal and the limitations on the possible upgrades you can install on a car whilst on your Provisional Licence in Aus are quite vast.
I don't know the greatest amount about performance mechanics either, just what I know from my mechanic mate, who is only an apprentice.
Hopefully this will be a great place to learn

 

Leave the crank pulley stock as it is actually the dampener for the Straight 6 engine harmonics. If you want the other pulleys smaller for less rotational mass then go for it. Just please leave the crank pulley stock or a new stock one.