I am building a GAE site that uses AJAX/JSON for almost all its tasks including building the UI elements, all interactions and client-server requests. What is a good way to test it for highloads so that I could have some statistics about how much resources 1000 average users per some period of time would take. I think I can create some Python functions for this purpose. What can you advise? Thanks.
You can get a free linux micro instance from EC2 and then run ab (apache benchmark) with lots of requests.  You can change number of requests, concurrent requests and you can even launch multiple EC2 instances from different data centers.
How to test my GAE site for performance