You could build something with a custom spec, but as Anzo said, it's not really a viable option for a business. My desktop for example has an AMD 965BE quad at 3.7ghz, 4gb of ram (soon to be 8), and two graphics cards. Everything that can be overclocked is, and it never stops breaking down. I must reinstall Windows 6 times a year because of driver issues and the like.
My two pence for anything business related? Stick with an Intel based pre-build. The drivers will probably be more stable because they're not releasing new ones every month to try and eak more performance out of the hardware, and the hardware itself will probably last longer because it's been assembled by someone who does it for a living.