My experience is that they are much hardier than 45F, even in pots.

I live in NW FL, Zone 8b. We get frosts and freezes here every year; most
years the coldest temps we get are somewhere in the mid-upper 20s, but some
years they go as low as the lower teens.

Sago palms (Cycas revoluta) are grown *everywhere* around here. I've got
several myself (some dating from 1992-1993), which I have *never* protected
from the cold. In the coldest winters, some or all of the foliage dies
(turns brown). We simply prune off the damaged foliage and they sprout new
leaves in the spring. In mild winters, the foliage usually takes no damage
at all.

Some people protect their especially nice (heavily branched and full) sagos
from all frost - but they do it for strictly cosmetic reasons.

But don't take my word for it:
http://ag.arizona.edu/pima/gardening..._revoluta.html

http://www.floridata.com/ref/c/cyca_rev.cfm

http://www.junglemusic.net/palms/cycas_revoluta.htm

http://www.sunpalmtrees.com/Cold-Har...Sago-Palms.htm

These are planted in the ground. Plants in pots would be more susceptible to
cold, but they can still take much more cold than 45F.

My mother, who lives several miles north/inland of me, kept several outside
in 3-gallon pots for about 3 years. Their only protection was provided by
the oak tree they sat under. None of them suffered any lasting harm (just
some frond damage), despite temps in the 20s each winter.

Also, the local plant retailers sell them year-round here. They leave the
potted sagos (1 gallon size and up) outside, unprotected, except during the
very hardest freezes. I've shopped many times after a frost or freeze and
seen row upon row of healthy green fronds soaking up the sunshine - very
cheery on a cold January morning.

Yes, Dallas winters would kill a potted sago. The OP should plan on bringing
it inside each winter. But I think he can safely leave it outside until just
before the first frost, not just until the nighttime temps drop below 50F.

Laura