There is a lot of individual variation. Also, women tend to sleeP much colder than men. And, you cannot really count on the sleeping bag rating -- unless it is a Western Mountaineering bag, I usually add 10-15 degrees (up) to the bag rating (meaning that I would not use a 32 bag, depending on quality and fill, below 42-45).
Here is the forecast for an area near TrailCamp on Whitney. Night time low temps range from 18 to 37 for the next week (elev. 12,182'). That is quite a range.
http://forecast.weather.gov/MapClick.php...75#.Uj8HfYl5mc0Speaking only for myself, I'd want at LEAST a good quality 10 degree bag for a nighttime low of 18, and I'd probably end up sleeping in a down jacket and heavy long johns even then. I'd want at least good quality 25 degree bag for 37. Keep in mind a forecast is just a forecast...it can get colder.
And you'll want a good hat to sleep in.
FYI, my go-to Sierra bag is now a 900-fill Feathered Friends 20 degree bag. Works for most of the year depending where I am. It weighs less than 2 lbs. If it is 35 or above, I just leave it unzipped. It is so good that I would consider using it down to 18 degrees -- with a good quality silk or fleece bag liner plus warm sleeping layers. (I do have warmer bags for winter mountaineering...when temps can dip to 0....or below).
A 32 bag is something I only would find helpful at higher altitudes during the warmest days of summer. And with I 32 rating, I'd be looking to be substantially under 2 lbs, but that is me. If you go for performance, it can get expensive.
I hope this helps.
P.s., any closed cell foam, Thermarest-style, or insulated air pad with should be fine -- look for an r-factor of around 4 or better. I find insulated Big Agnes Aircore pads to work well at a reasonable price (as a side sleeper, I like a 2+ inch pad), but that is personal preference.
