Hello.  I have a C521 desktop.  I have had the same video card in this pci-e slot since I bought this.  One day the video stopped working (computer worked but no output to screen).  I replace video card but same issue.  The onboard works fine (well it works, but not really "fine").  I have tried several video cards, several OS's, but I cannot get the pci-e slot to be seen by any system.  I also thought it was the power supply, but after testing, this works fine as well.  The only physical issues that I can tell is that my computer is running hot (even with it open).  I called dell tech support but could not get any help as I am leaning towards an issue with the GPU (same chipset that HP is replacing due to manufacturing issues - nvidia 6150).  Any ideas?  Oh, yeah, I tried some graphic driver updates/roll backs, not success.  Thank you in advance.