1. Yes if you want to use dual channel
2. 2 x 1GB would be your best option
3. It is best to install all ram of the same frequency. As you say it will run at slowest frequency.
4. Here's the hundred dollar question. It depends what you will be running on your computer. If you are doing video editing or something like that you will notice the difference. If you are doing basic computing tasks you will not see any benifits.
While Vista32 only sees 3.5GB it will do no harm installing 4GB.