Building your own computer uselly is almost cheapist. However with a prebuilt system you get the security that everything has been tested to work in the box.
I do NOT recomend DELL as there custmor service is horrible.
For a business computer i would recomend Intel. Try the new PD core with VIVE technology.
Gateway makes a nice VIVE system that is a PD 2.8Ghz with one gig of DDR2 memory. a 250GB HD and internal graphics most likely intel extream. But Gateway give you the PCI-E slot on the MB so you can upgrade it later if needed.
The only thing i dont like about this computer is the HD its only a 2mb cach and its kind of clunky.
Also remember that Gateway has been rated last by Consumer Reports. But dont use this one sorce to make your choise. A extended service plan might be a good idea on a Gateway.