In the PC world, it isn't too tough to get overclocking tools. They have existed for years and can, with cooling apparatus and other neat tricks, double the speed of a typical Intel CPU.
The Macintosh world and Apple's EFI BIOS replacement make overclocking more difficult - well at least less often attempted. A few products have popped up over the last few years, but they have been very limited.
Earlier today, ZDNet.de released a tool that looks very promising in this regard. According to their website, the tool can currently overclock Mac Pros and XServes with Intel Xeon processors. They use the example of taking a base model Mac Pro running at 2.8 GHz and getting it to run faster than Apple's top of the line 3.2 GHz machines.
This tool not only increases the processor speed, but it also speeds up the memory and bus speed as well. Thankfully, Mac Pros and Xserves have error correcting RAM which should help with reliability at these higher speeds.
Since this is a software overclocking tool, it doesn't pose too much of a risk, but as always in this arena, proceed with caution and I am not responsible for imploding machines. Bugs in clock time have been reported but fixes are coming. To go back to your old clock speed, you'll just need to reboot.
If you are looking to upgrade your base model Mac Pro into a top of the line model and don't mind the inherent risk, perhaps you should give it a try. I don't have any machines that fit into the Xeon category that I can test this with but let us know your results.
Like this Apple blog? !