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#51
Going to weigh in here with a quick question. You have a 5770 in that Dell, does it by chance happen to have the stock 350W power supply in it?
Dell Studio XPS 8100 Desktop Details | Dell Canada
If so I hope they have done what is mentioned here with the same XPS 8100 and a GTX 260,
Studio XPS 8100 Power Supply - Okay to run GeForce GTX260 ? - Desktop General Hardware Forum - Desktop - Dell Community
and I have found a few hints that they may do the same thing with the 5770. If they have gone and set the card to run with lower clocks it was most likely done in the drivers (INF mod), and installing the reference AMD driver will have removed the artificial limit on the clocks the card runs at, which if that is the case and it only has the 350W power supply could quite possibly be the cause of your problems.
One way to find out for sure if that is what they have done is to uninstall any driver from AMD for the card and install the driver from Dell,
Drivers and Downloads
then grab this,
GPU-Z Video card GPU Information Utility
run and take a screen shot of what it shows, then uninstall the driver from Dell, install the AMD driver, and then run GPU-Z again and compare what it shows.