Replacing your old drafty windows will help keep heat inside your house and prevent the cold air from seeping in.
You'll certainly see lower heating bills since your furnace won't have to run as much.
If you can afford to get the windows replaced I'd recommend doing so as you'll save money on heating bills enough and more to pay for the new windows and installation of the new windows.
And you'll be warmer in your home without all the drafts coming in. You'll certainly notice a huge difference in how warm your house is and a big difference in your heating bills after upgrading your windows.