USB vs PCI cards for wireless network

We are planning on purchasing two new Dells (8400s with XP Pro) with a wireless internet connection. When we spoke to our Cable company two different technicians strongly advised us NOT to use USB wireless cards, but to go with PCI wireless ethernet cards to set up our wireless networking system. They say that drivers from other peripherals using USB 2 connections often interfere with the USB wireless cards, and we are likely to run into trouble. They are getting many calls with just such difficulties.

The problem is that the Dimension 8400s have only 1 or 2 usable PCI slots, depending on what video card you get. So if I use a PCI for my wireless card, I will have nothing left. Does anyone else here have any comments about the use of USB wireless cards vs PCI slot wireless ethernet cards? Thanks for any guidance - we are pretty new to wireless networks and are somewhat clueless.