AMD 7750 gives monitor Black Screen

Make sure all necessary power plugs are plugged in (6-8 pin from PSU if necessary). If you haven't installed the drivers yet, you may have to do that as well as update Direct X, if necessary, and if your motherboard/CPU has integrated graphics, try using that to boot into OS and update drivers if possible?

I would try it, because depending on what you had before, it might not read properly. If that still doesn't work let me know, but make sure you update Direct X as well.

Assuming you're using on-board graphics, if the card is not connected, then that makes sense. Have you already ensured you have enough wattage from the PSU?

Right, I forgot that was mentioned above. The PSU has 460, but do take into consideration that the other components may take up more than that extra 60 watts combined, but it may not be the main issue. Have you tried using a different connection to monitor?

Also, is it possible to post some specs as well?

Your XPS can handle GTX660, so it is not "strong card", it is driver issue.
You say that old AMD 5450 is working OK?
Next,

Second

1. Go to Add/Remove Programs and uninstall all AMD drivers look here for detailed instructions http://support.amd.com/en-us/kb-articles/Pages/GPU57RemoveOldGraphicsDrivers.aspx

2. Uninstall the Catalyst Install manager (Express Uninstall)

3. Reboot the computer and log back into windows as normal

4. Go to folder options and select “show hidden files and folders”

The next part of the guide will remove all ATi/AMD files and folders from the computer.

5. Go to your C drive and remove the “AMD” folder

6. Double click the “Program Data” folder (Normally hidden) and remove all ATi/AMD folders

7. Go back to C and double click “Program Files” and remove all ATi/AMD folders

8. Go back to C and double click “Program Files (x86)” and remove all ATi/AMD folders

9. Go back to C and double click “Users”

10. Access the name of your windows profile and double click “AppData” (Normally hidden)

11. Check the “Local”, “LocalLow” and “Roaming” folders for any ATi/AMD folders and remove them.

12. Go back to “Users” and check the “Default” folder (Normally hidden)

13. Double click “AppData” (Normally hidden)

14. Check the “Local” and “Roaming” folders for any ATi/AMD folders and remove them.

The next part of the guide will remove registry entries left by the AMD drivers.
15. Close everything down and go back to your desktop

16. Click start, then type “regedit”.

17. Click “HKEY_CURRENT_USER” and extend the folder

18. Click “Software” and extend the folder

19. Remove all AMD and ATi Folders

20. Click “Wow6432Node” and extend the folder (Within the software folder)

21. Remove the “AMD” folder

22. Click “HKEY_LOCAL_MACHINE” and extend the folder

23. Click “SOFTWARE” and extend the folder

24. Remove all AMD and ATi Folders

25. Click “Wow6432Node” and extend the folder

26. Remove all AMD and ATi Folders

27. Click “HKEY_USERS” and extend the folder

28. Click “.DEFAULT” and extend the folder

29. Click “Software” and extend the folder

30. Remove all AMD and ATi folders

31. Click “S-1-5-18” and extend the folder

32. Click “Software” and extend the folder

33. Remove all AMD and ATi folders

34. Click “S-1-5-21-1632250243-966907716-928185508-1000” and extend the folder

35. Click “Software” and extend the folder

36. Remove all AMD and ATi Folders

37. Close down regedit and empty your recycle bin

38. Undo the folder options change to hide the hidden folders

39. Reboot

To avoid extra headache with installation, do manual instal:




​A manual installation of the display drivers can be performed through the Windows® Device Manager:

* Click on the “Start” button, type “device manager” in the search field and click “device manager” from the search results
* Expand “Display Adapters”, right-click on the AMD Radeon™ entry and then select “Update Driver Software"
* Select the “Browse my computer for driver software” option.
* Click “Browse” and navigate to C:\AMD\Support\[Catalyst™ version]\ and click “OK”. Note: This folder is usually extracted by default to C:\AMD\Support in the initial running of the AMD Catalyst™ software suite
* Click on “Next” to install the driver software and restart the computer after completing the driver software update

I hope you will be fine.

Anything else I can help you with?

This just doesn't make any sense, unless you got 7750 lemon....

If your XPS is working good otherwise with old card - it is not PSU. 7750 reported working on even 165 watt PSU!
So your could not degraded by approximately 300 watt. Something else to blame.

Do you have any friends with AMD video cards - you could just insert yours into their PCs to test the card.

Not sure what else can be done...

Can you borrow GTX card to try to install? You need stronger card than yours to see where is the problem>