Importing a site - how intensive is it for the server?

From my experience I wouldn't be concerned about processor or memory performance, I'd be more concerned that you have set your site up right to the export and import work properly. I've imported a site with 2000+ pages without problem, it took roughly 5-7 minutes or so but it worked fine.

Did you follow the Kentico best practices on your file/folder structure?

Thanks Brenden - great to hear. File wise, yup, we are in good shape. I guess the Import to Staging will really tell though.

Another quick one that I don't see covered in the documentation.. will all the environments "know" about the "sites" in each environment simply through the Import? IE.. when I export "Site #2" to QA and Production.. will QA and Production know to sync "Site #2" through Content Staging?

From my experience, no. You will have to setup content staging on each individual server. In the past, I've updated staging and test with a copy of live before I start a major overhaul. In this case, I've documented what needs to be updated from a license, url alias, web farm servers, content staging, scheduled tasks, etc. I've found this backwards promotion to work very well. You might check to ensure all your Site Manager>Settings are using specific site settings vs. inheriting the global settings. This has bit me in the past. I've set global settings and assumed all would be good, which they were until I imported into an new install/instance and those global settings were different or not set.