Resistances
I seem to recall someone doing a study on these a while back but I can't remember... Anyways, I just did some experiments with wands of lightning and rings/cloaks of electricity resistance. I was bouncing lightning bolts off walls two spaces away from myself. I found some interesting things... With no resistance I consistantly took 18-19 damage per shot. With 15 resistance (one ring) there was a dramatic, but irregular drop -- damage taken now ranged from 1 to 17, tending to be closer to 8 or 9 points. With 30 resistance (two rings) I took nearly no damage. I spent probably 16 shots, and took only 3 points from the first one and 1 point from a later one. Other than that I took no damage at all. And finally, with 55 resistance (2 rings + cloak), damage appeared completely negated. Note that I did have 1036 base HP. Some stats may influence the damage. I would suspect that if any did, they would be endurance and/or agility. I'm still confused though. I can't see a trend to the damage reduction. The closest thing to a geuss that I can come up with, is that damage is reduced by Resistance - RAND(Resistance / 2). I don't have the source on my HD anymore; someone who does have it could look this up to verify... Or do your own study. I was going to test with acid as well (by drinking it) but I got bored. Another think I'd like to check is to see how much resistance increases on cloaks with enchantment.

Natural HP regeneration is based on a percentage, I believe, so when you tested with higher resistances, much was probably recovered straight off. Try the same with 300 HP, you'd probably get different results.

repeated experiment with fireballs. seems that a single fireball can do more that 30 damage to a completely unresistant creature, while with a single ring on, it does around 9 damage. the figures look wrong somehow.

It's not due to natural HP regeneration. I know, because I just made similar tests while polymorphed into a skeleton (and statboosted to have 300+ HP). Skeletons don't regenerate (like golems, but golems can't zap or use rings etc). But, from the looks of it, it's a very good idea to pack resistance rings. I'd like to see some tests for acid resistance. A while back I recall someone checking the resistances on plants (which for all intents and purposes appear completely immune to acid); I believe they had ~50 acid resistance. I can't remember exactly. Unfortunately a quick check in the script files didn't show teh exact number, so someone'd have to source dive to find it...

It's not due to natural HP regeneration. I know, because I just made similar tests while polymorphed into a skeleton (and statboosted to have 300+ HP). Skeletons don't regenerate (like golems, but golems can't zap or use rings etc) Actually golems can zap wands, but yeah, my bad. I suggest you keep testing with skeletons so you don't have to worry about natural HP regen.