Confusingly, the PC91 was in fact an 8-speed chain. You're right that in the present line up, the first digit signifies the number of gears, so the PC830 is a low end 8-speed chain and the PC991 is a high end 9-speed chain. SRAM have used this logical scheme for a good while now, but when the PC91 was the top-line SRAM chain about a decade ago the situation wasn't quite so clear! I suppose they inherited the confusion from their takeover of Sachs and sorted it out later.
The PC-91 will work fine with any SRAM 8-speed powerlink, but not with a 9-speed or 10-speed one. So the link you got with the PC890 will work fine. Any decent LBS should have decent quantities of 8-speed (silver), 9-speed (gold) and 10-speed (black) powerlinks. In my opinion the SRAM powerlink is the best solution on the market for quickly splitting and rejoining a chain. CRC has them all here:http://www.chainreactioncycles.com/Mode ... odelID=871
As for whether the PC91 would be an upgrade over the PC890, well that's in large part dictated by the other components. As I said, the PC91 was the top-line 8-speed chain in its day, so provided it's not too badly worn then it should perform very well. The PC890 is a fine chain, but if it's brand new then it might not be tolerant of wear on the cassette or chainrings. The shifting might be a bit smoother on the 890, but I'd guess that the 91 is lighter, if that's important.
I guess the chain in question is the one I included with the XTR cassette - I don't think that it's excessively worn and it worked very well with the cassette. How it works with the chainrings remains to be seen. I cleaned it very vigorously a while back so would benefit from thorough lubrication. Perhaps soak it in a light lube bath to get it right into the rollers, and wipe all the excess off the exterior parts after.