G5 performance measured against competition
If you don't mind reading about performance and performance
only,
this anandtech.com article's for you.
There are two things very important (and sad at the same time) for me: First one is that the 2.7 GHz machine (almost a year after) is really not much faster than the 2.5 GHz machine. And the other one: For server tasks like Apache and MySQL, Mac OS X is simply not a good option. Their sum-up reads:
"Mac OS X: Workstation, yes; Server, no." - And quite certainly, after reading their document, it's true. Then again, you might choose Mac OS X Server for its functionality to cater for Mac OS X Client machines. But that's not what the linked article is about.