Nimbostratus
Cirrocumulus
First off, whats you'r traffic? If it's http/https, then you can use loadrunner or similar (Or canned scripts utilising curl, wget or whatever). If it's LDAP, I've had good luck using custom programs (e.g. charge - works fine on Linux, needs a few tweaks for something like solaris). There's also slamd, but that tends to crash and burn too much under load for my liking.
After you've found your testing tool of choice, you need to find a way to measure throughput and response times... That's another can of worms. There's a sample iRUle for measuring LDAP response times on the codeshare that I uploaded last year. Jason is working on formatting the rest of the articles describing the work I did last year for a bank here in london on measuring throughput, and making decisions based on response times, so they'll appear in the tech-tips once he gets some spare time. It's not purely about testing performance under load, but it does provide a reasonable platform for doing so once it's all setup.
H