As mentioned, I wouldn't go higher than 6/4 for street. At the moment in my GT-R I'm running old TEINs (no damper adjustment), which I estimate are around the 7-8kg/mm mark. Very good for corners, but for Sydney roads they are way too stiff.
My Ohlins with Eibach springs on the other hand, I estimate them to be around 4-5kg/mm, a lot more comfortable but still decent for handing, has body roll though (softer compression on the Ohlins compared to the TEINS). Nismo S-Tune spring rates apparently are 5.3/4.8 for the R33 GTS-t.
If in doubt, get these: http://www.otomoto.com.au/p/8475209/hsd-ad-type-coliovers-for-skyline-ecr33.html, 7/5 kg/mm with damper adjust. Good for full weight Skyline that you will track occasionally IMO.
A lot depends on how much harshness you can tolerate. I'm seriously considering going back to my Ohlin/Eibach suspension as the TEINs are just too harsh for Sydney roads (35 series tyres don't help either ).
A mate of mine got the Blitz set from Just Jap for his S14. They are 8/6 but more comfortable than my TEINs. S14s are obviously lighter as well. Food for thought.