I'll echo the suggestions for the 20-degree bag and make a suggestion for a killer bag at such a low price that you will be able to also buy the Costco Kirkland down sweater and a really large tri-latte delight at a yuppie coffee joint: the Kelty Cosmic Down 20.
It's reviewed on this site in the BPL reviews and in several threads. You will find it under $150 and it is excellent. Several of my Scouts and adults who've spent time in the eastern Sierra (Garnet, TI, Clark lakes,Kearsarge Pass, PCT, JMT) with me will offer their endorsements whenever asked: it's warm, it's light and it's very affordable. When the weather turns to "surprise mode", it's a great bag for 3-seasons.
The Cosmic Down 20 even comes in a shrimpy-size for midgets under 5'5" that weighs 2 lb, 3 oz... and can be found for as low as $70 on sales right about now...