getting new spark plugs hopefully...heat ranges?!?

[SideWinderGX]

k, test driving the 87 supra tomorrow then hopefully buying it on saturday. im obsessed with this car (probably not a good thing. i dreamt about driving it last night -.-) and was thinking about what i should buy first to make sure it runs great. obviously the headgasket problem will be sorted out very soon, it hasnt been done yet. HKS 1.2 mm gasket and arp studs, if i remember correctly (its in my other topic, ill look back again). but anyways, ive settled on some silicon hoses to replace the rubber ones for look and performance, and spark plugs.

iridium seems to be the way to go, but whats all this heat range stuff? im so confused haha. japanese plugs go up and american heat ranges go down, and you go down another level per ~85 HP, i dont know. could anyone explain this in lamens terms and tell me which are a great plug to buy? thanks haha