Well, I'm a little surprised no one has mentioned the recent OS news, and how it affects this discussion. Leopard requires a GHz G4 minimum to install. My office G4 has a 733 MHz G4. Further down the road, Mac OS 10.6 will only run on Intel processors, but that is over two years from now most likely.
For me, this news means I've got a purchase decission to make in the near future. I can get along with Tiger for a while, but I'll likely buy another Mac within six months. Will it be a new Mac prosumer tower? I have my doubts that Apple would build one by then, but if one existed that would be my first choice I believe -- not for the office, but to replace the G5 PowerMac for gaming and software development. My dual 1.25 GHz G4 that runs Logic Studio 8 will be okay for a few more years.
Since Apple shows no sign of building a prosumer mini tower Mac, I will be buying a used MacPro from eBay. Now some may say I should get an iMac for the office. Yet, I don't think that's a good idea. I want a large screen display, so it would be a pricey iMac, and I don't need current iMac performance in the office. A used MacPro is a far better choice, and then use it for games and software development.