Insulation for outer walls when replacing windows – GreenBuildingAdvisor

Newbie here but lurking here for some time and first Question in this forum!

Apologies for any mistakes  – If I’ve missed providing any useful information, please do let me know!

My house in San Jose, CA has single pane windows and seemingly bad insulation ( No insulation in the outer walls , little to no attic insulation ). Summers are very hot and heater bills are high in winter. The attic is also very hot ( when I stick my head into the garage attic and the main house attic both feel very hot ). 

I have a few questions  :

1.  I’m getting them changed to double pane windows . How much improvement should I expect ? 

2. My contractor is  removing the stucco and there’s no insulation in the outer walls. Should I get insulation in between the open studs  ? What type of insulation should I put in ?

3. Should I put in attic fans to help with the heat ?  Should I put in a powered / unpowered fan ? What kind of fan would be a good choice ? 

If I’m thinking about this wrong, please do let me know!

Thanks in Advance!

-Harry