Determining a realistic measure of requests per second for a web server
If you care about your server's performance when accessed from somewhere in the world, ask a friend somewhere in the world (should have good bandwith) to install sproxy + siege on his linux box. Just download, configure, make. These tools are small, they compile within seconds.
First of, start sproxy on the linux box. By default, it will run on port 9001 on localhost (127.0.0.1). If you want to access it from outside, just pass it the outbound ip address as a parameter.
Now connect to sproxy by setting your browser to use this ip and port as proxy for HTTP. Everything you do from now on is recorded by sproxy and can be replayed later. Now surf around on your site, do things your customers would do and try to do "expensive" things which use your server.
When done, end sproxy by hitting CTRL^C. It recorded your actions to $HOME/urls.txt. Move the file to where siege resides. To start stress testing, run siege -f urls.txt -d NUM -c NUM. d stands for delay between requests, when doing performance tests, use 1 (second). c stands for the number of simulated concurrent users. Choose at will, but start low.
Siege will show you the number of transactions per second, errorrate, how long average requests took etc. It is a powerful and easy to use tool.
If you need more info on parameters (there are many), check the siege manual an the sproxy manual
To get more realistic results, let many people test your server from various countries at once and let them send you the statistics.
The realistic requests/sec measure should be taken from the access logs. IMO, the request latency has nothing to do with server load, as the server processes all the requests at the same speed irrespectively of their origin.
Consider using services such as Soasta Cloudtest. With it you can get pretty detailed reports on your tests and you can run performance tests from various public cloud/virtualization providers. You can configure how hard and for how long do you want to hammer your servers. They also have a free "lite" version so you can see what it can do before committing any money.