I am typing this on my new xps420 that I ordered April 6 along with my dual ATI tunners. While the 420 shipped right away, the tunners showed a ship date of 5/23.
So I called Dell to get some more info on the 21 day return policy. They were very helpful, and I explained that I wanted to test the whole system together before deciding to keep it or not. The customer service rep, Roy, said that not only would he notate my order so that I could return the entire system after testing my cable cards, but he also checked to see if he could move my tuners' ship date up.
Today he called me back and told me they are scheduled for delivery this Thursday! Very cool, and I have to say that Dell definitely impressed me with the customer service.
I would recommend giving them a call and explaining your situation. It def helped me!
Dell XPS 420 - Vista Ultimate x86
2 x ATI Digital Cable Tuners
HDHomerun Tuner
1.75 TB storage
Xbox 360 + 40" Sony Bravia XBR4 LCD = Living Room
Xbox 360 + 50" Samsung Plasma = Media Room
Linksys DM2200 + 27" Sony Wega = Bedroom