20% savings seems a bit hopeful. At $800/window it's gonna take a long
time to realize those energy savings. I don't know how many windows the
OP has but I'm sure to do them all would cost $10k atleast, even if you
save $2-300 per year, that's 30-50 years to just recover your initial
outlay.

I'm in a similar situation and I am not replacing the windows. I do
have storm windows and they do help.

I'm going through, window by window taking the sashes out, removing all
the old paint from the window frame inside and out and the sashes.
Removing the old glazing and the glass (very carefully), priming
everything (with oil based primer), re glazing and painting (with
Latex) and adding weatherstipping. Replacing ropes with chains,
cleaning the pulleys etc etc. It's not quick and it's not easy, but
it's not rocket science either. To see the bare wood of the sashes and
the wooden pins in the joints and realize how long they've been around
is just amazing. The windows fit the house and if that is important
keep them. The windows I've done don't rattle, glide nicely and look
great.