Well, I don't think he would have had to make it in the cracks of doom. That was really just Sauron's smithy - a very powerful though. The elven rings weren't forged in Sammath Naur, but in Eregion.

I think maybe Saruman's ring did have some power in it, though not much. (It was a sign of pride that he flaunted his ring (while the three were hidden), and ironic that he announced this to Gandalf, to actually had on of the three (though Saruman didn't know, or he probably would have tried to sieze it).) Just think of Aragorn, Legolas, and Gimli chasing the orcs. They keep mentioning how some power gives strength and speed to the orcs, while holding them back. This ability may have been within Saruman's innate power, but it seems quite powerful. Gandalf is only able to give such hope to the Gondorians (and the troops of Erkenbrand) because of Narya (and even that power has a smaller range than the power Saruman used to thwart the three hunters).

It seems to me then that this is evidence that Saruman's ring did have some effect of increasing his power.

Of course, Saruman's ring would have been under the sway of the one ring, just like the others, and bouncd up with the fate of all of them. It's power would have thus passed with the destruction of the one.

Cheers.

Dave C-Q