Another way of looking at is as a plot device level artefact. Something beyond numbers. Something on the level of the Summer Lady or maybe even Mab.
Powerful items can rarely translate well into balanced games. Even relatively minor ones like Saberhagen's 12 Swords. None of them were as powerful as the One Ring, but each of them were "rule breakers". Hold Townsaver in your hand while defending a town (or at least a building with women and children in it) and you became an unbeatable swordsman. Even if you were overwhelmed and critically injured you wouldn't feel the pain until the battle was over. Then you might fall over dead, but you won the battle.
Or Farslayer - recite a verse, throw the Sword in the air, and it would fly off and kill any enemy you named. The target could hundreds of miles away inside a steel vault and he would die. Of course the sword would stay in the corpse and if your victim's friends guessed who had used it then they could throw the sword back at you and you would die. Nothing earth shaping like the One Ring, but it's effects are almost impossible to map to any game system.
In Lord of the Rings the forces of good had four "wizards" (spirit beings sent to aid the mortal races), the Three Elf Rings, Elf Lords, armies, fortresses, and if Sauron ever got his Ring back then none of that would matter. If Gandalf, Galadriel, or even Elrond had claimed then all of that could have been added to Sauron might and it wouldn't have mattered - if a being of power had the Ring it was game over for everyone else. That's a "you win the game" type thing.
Here's how ICE statted the Ring in MERP (the benefits, not the bad stuff):
The Ring takes (600 - wearer's level) days to Master.
It multiples the amount of magic energy you have by 18 times.
It can control or resist any device made with the Ring's aid.
It controls all wearers of the Nine Rings of Power regardless of range (if mastered).
The wearer can cast spells at three times normal range or against any target the wearer can see (even when the target is aided by some device) - whichever is greater.
The Ring-wearer's spells, physical attacks, and maneuvers cannot fail.
Um, that last doesn't mean "always hits". In Rolemaster, if you failed a roll sometimes bad things happen. This result is more of "inflict 0 damage while taking no damage" result.
The only way I can think of translating that would involve granting 18 (or more) refinements of the PC's choice - which means picking it up is enough to turn virtually any PC into a NPC.
As for what Boramir thought he could do with the Ring, it lied to him. Yes, the evil, bad, nasty Ring lied so that Boramir would take the Ring closer to Mordor where Sauron could have found it and said "Mine! I call yank!". Let's face it, the Ring was bored of hanging around with Hobbits and proto-Hobbits. It wanted back in the game and if Gandalf, Galadriel, etc weren't going to use it then it wanted to go home to Daddy.
Oh, and if people want to know more about those Sword, head to
http://en.wikipedia.org/wiki/Books_of_the_Swords - but beware the spoiler beast that lives there.
Richard