If you haven't played through the game don't read the answers to this post because I'm asking for a spoiler.
Where is the ring of regeneration? I haven't played in a while but I'm sure it wasn't where I usually find it. Does anybody remember?

Waukeen's Promenade (Adventurer's Mart) - Can be pickpocketed off of Ribald Barterman

Asylum (Dungeon Level One) - Reward for completing the "stone face" riddles

Those rings are awesome. If you wear 2 of them on 1 character, will they be cumulative with eachother? So you'd get 2 HP in the time period that you would regen 1 HP with just 1 ring?

There is a better ring in the game.

Yes they will cumulate. Like regeneration items stack, unlike regeneration items do not stack (with the exception of the Lilith Amulet if you have tactics installed). Instead, the regeneration is based on the most powerful item.

Possible item spoilers
~
~
~
~
~
~
~
~
~
~
~
2 * Ring of Gaxx = total 6 HP per round
Ring of Gaxx + Ring of Regeneration = 3 HP per round (not 4!)
2 * Ring of Regeneration = 2 HP per round
Axe of the Unyielding +5 & Ring of Gaxx = 3 HP per round (not 6!)

Also worth noting that natural regeneration (due to a CON score of 20+) does not stack with regeneration provided by spells, potions and items.

What is the rate of natural regeneration (once at 20 CON)? If it depends on your Constitution and you know the rate at each CON level, please let me know.

20 CON = Regenerate 1 HP every 6 rounds
21 CON = Regenerate 1 HP every 5 rounds
22 CON = Regenerate 1 HP every 4 rounds
23 CON = Regenerate 1 HP every 3 rounds
24 CON = Regenerate 1 HP every 2 rounds
25 CON = Regenerate 1 HP every round

is there a way to increase your CON? i mean like a ring, a girdle or a pair of stinking boots?

just wonderin'

<font color="gold">There are only two items that permanently raises your Con that I can think of, and both are in ToB. Other than that, there're some spells that raises your Con, like Draw Upon Holy Might. But this isn't a permanent change. ;)
There's also a few other ways to raise your Con, without using items. ;) </font>

There's a girdle of 18 Con you get from Unseeing Eye quest (Gaal has it I believe), but that's not enough to give you regen :/