Lloyd Chambers has had a Web site that helps Macintosh users understand how to get the most performance out of their computers for some time now. He's gone a bit further, though, and hooked up with one of the better Mac accessory dealers, Other World Computing, to provide built-to-order computers configured to maximize performance. You can, of course, do this yourself by following Lloyd's suggestions and doing all the disassembly and replacements yourself, but this new service basically gets you the Mac you want without breaking out the Torx set and playing find-all-the-screws-and-plugs.
Short answer to maximizing Mac performance: replace any 5400 rpm drives with 7200 rpm server-tested ones, strongly consider using an SSD for the boot drive holding OS and applications only, maximize RAM, and for desktops get an eSata card for external drives plus use a small fast drive for Photoshop scratch space. If you're going to max out the drives in a MacPro, consider double-RAIDing them (two RAID 0s that are linked as master and backup). The performance differences in doing the above tend to far exceed faster CPUs or even updating an older Mac to a newer one. In other words, do these things first before thinking you have to buy a new Mac.
And speaking of new Macs, yes, there are performance boosts with the latest iterations, though they are in some ways modest in nature. Raw conversion seems to be one place where substantive gains can be made, though. MacBook Pro users should also seriously consider the i7 processor option instead of the i5.