If you have a bad knee, are overweight, out of shape, older or simply not used to climbing, do yourself a favor and use a triple 24x36x46 or perhaps a 34x44 if you have a (110bcd) double crank. I'd use a 11-32 or 34 tooth rear. This will give you somewhere around a 20 inch low with the triple and a 28 inch low with the double. It may not look racy but its what smart cyclists ride when they have big hills. I am a 49 year old cyclist with a bad knee and I live in an area (foothills of Mt. Rainier) where there are major climbs. When I was younger and weighed 168 pounds I could climb most of them in a 40 inch gear. Now that I am old and feeble, I need a 20 inch. Most of my rides are between 10 and 40 miles in this terrain and what I can do at the beginning of a ride is different than at the end.
Moral of the story.........you can always use a lower gear. I don't think high gears are very necessary since you can often coast down hills at up to 60 mph depending on the grade. Its easy to spin out a 120 inch gear around 35 mph +/- and most realistic riding is in the low to mid range.