Your not getting what I am saying. When I say standardized PCs I mean all the PCs the same. You buy some used machines from a used computer store you may get 10 with ATI cards, 10 with Nvidia cards, 5 with Intel graphics, 3 with Intel NICs, 15 with 3Com NICs, 2 with Linksys NICs, 4 with Creative sound cards, 12 with integrated sound. See where I'm going? Keeping the machines up to date becomes a nightmare not to mention imaging of the machines is troublesome at that point. No need to worry if an Windows update is going to bork the drivers for some piece of hardware that 4 machines use.
What do I care if they don't use standard motherboards or power supplies? I buy anything for a business and it gets a 3 year warranty. After 3 years the machine is not worth upgrading. We have over 450+ machines in our company. 75% of them are Dell Optiplex systems (G1, GX1, GX110). You know how many failed motherboards there have been? 0 Know how many failed PSUs? 1 because of a power spike on the circuit. The parts that do fail in them (hard drives, fans) are all common parts and even if it's not a quick call to Dell and the part is here the next day!
The management functions of Dell machines alone are much better than you can get on a mix of clone machines. With the Openmanage clients on the Dell machines you can view the health of the hardware from a single console and know about failing hardware before a user does!
As far as speed. For 90% of our users they do Office, IE, Acrobat, and Siebel. Do they need an AGP card? No. Intergrated graphics are fine for them.
If managing clone machines was easier to do than buying some Dell machines you would see all companies doing it!