The last thing I want to talk for performance testing about is tools. There are a lot of great tools out there, both commercial and open source. Do some Googling to see what's available, and what other people think of those tools.
Spend some time evaluating tools before you make a decision. Make sure that the tools can provide the information you need in the format you want before you pull the trigger.
While you're running your load tests, you're going to need to keep tabs on your server's behavior. Make sure that the tool you're working with can monitor the server's resources as unobtrusively as possible. If your tool of choice doesn't have server monitoring capabilities, then you can use a tool built into Windows called Perfmon. Perfmon lets you keep tabs on many things, but at the miniumum, you'll want to track CPUUsage, Available MBytes (the total amount of RAM available on the system), Bytes Sent and Bytes received. This will give you a flavor for how your system's doing overall. You can also track items related to SQL databases or the .NET framework, so look through the list of what's available and see what meets your needs.
As you can see, there's a lot involved with performance testing, and these posts have just touched the surface. The thing to remember here is that performance testing is *very* different from functional testing. You'll need to learn the difference between a 302 and a 304 http return code. You'll need to understand what parts of your application uses system resources and why. If you've never done performance testing, be ready for a pretty steep learning curve. Make sure you budget time, whether it's on or off the clock, to learn as much as you can.
I'm not a performance testing expert by any means, so now I'll point you to the people who are :)
Corey Goldberg has a great blog and builds performance testing tools. Definitely check him out:
Scott Barber is a perfomance testing guru:
Bob Dugan is an extremely smart performance tester and a great guy to work with:
Here are a couple of helpful books on performance testing:
Performance Solutions: A Practical Guide to Creating Responsive, Scalable Software
Performance Testing Microsoft .NET Web Applications