Stress testing by definition means to push the item under test until it breaks. I assume you mean you want to test your web site with 10, 100, 1000, or 10000 requests per second (or some settable speed) and see how it holds up or breaks. Selenium RC runs at the speed it runs, and can't be sped up.
For true load and/or stress testing of a web server, I suggest you look into a product called Avalanche. This however is neither free nor cheap.