the denso and HKS are nearly identical plugs,your are best off going for whichever you get cheapest.plugs should be changed about every 18,000 miles.if your car is modified go for a colder plug not sure what parts numbers are.
hope this helps
justin, thanks for the valueable infos!! btw, some says plug no. 8 is longer than 7 (besides colder), is this true?! if i were to buy Denso Iridium Racing plugs, which is the exact model i should get?! any type G or etc. and what about the head size?! what does G signifies?!