Hi All.
Got it working now with ATi Windows 7 drivers the issue with the genuine ATi drivers is that dell change the hardware ID of the video cards and they are not recognised correctly by the ATi installer.
I have resolved this issue by flashing the cards with the latest Sapphire HD3870...
yes it does work.
Dell XPS 630i 2 x HD 3870 install old vista drivers first to get x-fire working then upgrage to the ATI beta drivers for 7 got it working on both 32 and 64 bit build 7000 for 32 and 7100 for 64.
could not work out how to upload a screenie but mail me and i will send it