How to stress test a SQL server? I've been tasked to stress test our MSSQL Server and MySQL Server. I would like to know if there are any tools or scripts which I could use on our current systems and the new system to compare performance?

I would like to measure read / write to disk and processor performance. Anything else that may be useful would be great as well.

Thanks!
Quest offers a free version of their Benchmark Factory that works for Oracle, SQL Server, and MySQL:

http://www.quest.com/benchmark-factory/

I recorded a video on getting started with it back when I worked for Quest:

http://www.toadworld.com/Videos/tabid/831/TID/559/cid/161/Default.aspx

You can call your own stored procs, replay traces, run TPC benchmark scripts, and more.  It's not exactly easy to use, though - thus the video.