My humble advice
Great choice in the 640!
However, I was thinking along the same lines that for a newb... a snubbie wouldn't be the best choice for home defense. I doubt distances in a bedroom would make a huge difference, but to become proficient and obtain accuracy, it is something that needs a lot of frustratiing practice at the range.
The lightweight of the 640 excels as a concealed carry, whether in a purse, on ones person, or in a vehicle.
Vs the 642, this is by far a better choice because the extra few ounces helps make it an enjoyable experience at the range where you should be visiting often.
A 640 and 100 rounds at the range might be a bit much for some females.
Certainly would hold onto the 640, but would add a heavier and slightly beefier K frame to your mix of handguns.
For home defense and as a good range gun for training, anything in a K frame would be a great choice. The L frame are equally preferable but are a bit on the heavy side for some ladies. Something with a barrel of 4 inches is pretty standard.
You have initially made a great choice and would build upon that with a K frame... A Model 10 (or 15, 19, 66, 65, 64... among the many variations) would be a great compliment!
I've tried everything from Glocks to Kel Tecs and returned home to revolvers.
My small collection consists of two jframes (442 and 649). My home/range gun is a L frame 586 (and have owned and sold any number of K frames).
Probably the best addition to my small battery is the humble Model 17. Hundreds and hundreds of cheap .22 caliber ammo on this K frame has done wonders for my enjoyment ans improved skills.