When a Form has been submitted and the "follow up with" is set to external url, can I make the url launch in a new window?

I don't believe so; I think that just takes them to the corresponding page that you enter. However, there may be a way to code that into your website/page or make a custom script? Anyone else know?

Definitely possible. Just select "stay on same page" as the follow-up for the form. Then, look at example 4 here.

Use the onSubmit() function and all you need is one line of JS in that function:

window.open('http://www.google.com', '_blank').focus();