EDIT: While I've been researching and composing this post, I see you've since stated that you had the GeForce GT620 option already installed. Please read the following to see if this may apply to why you haven't been able to install your GeForce 650 Ti:
I've done some research because I was very curious and your problem doesn't appear to be isolated, given the number of posts on the web regarding add-in graphics cards not being recognized on many new factory computers; not just Dell either.
From what I've been reading; the problem with third-party add-in cards not being recognized seems to be that Dell, HP, Lenovo, etc. are shipping their PCs, with Windows 8 pre-installed, in UEFI mode with secure boot enabled. It's the secure boot setting that prevents being able to install add-in cards (not just graphics cards).
Although the ability to disable secure boot is mandated by the Windows 8 certification requirements for x86 and x64 OEM computers, Dell in particular seems very remiss in including any documentation on just how to do that.
If Windows 8 came pre-installed on your computer, it almost certainly boots in UEFI mode. To get your computer to recognize your graphics card, youll need to switch the boot mode from UEFI to Legacy and then disable the Secure Boot option.
- Power on the computer and when the F2 Setup, F12 Boot Options appears in the lower-right corner of the screen, press F12.
- When the Boot Device Menu appears, use the arrow keys to choose the BIOS Setup option.
- In the BIOS Setup Utility, move to the Boot tab and disable the Secure Boot Control option.
- Next, navigate down to the Boot Mode option and switch the boot mode from UEFI to Legacy. Press F10 to save your changes.
- Once your computer restarts, press F12 to bring up the Boot Menu again and under 'OTHER OPTIONS' you should now see 'Peripheral Device Setting (OPROM Setting)'.
- Use the arrow keys to choose Peripheral Device Setting, then make the approptiate changes in BIOS to disable the onboard VGA and enable the PCIe graphics card. Press F10 to save your changes.
- Allow the computer to restart and test to confirm that your grapics card is now functional.
- Once satisfied, restart your computer then go back into the BIOS and switch the boot mode back to UEFI with secure boot enabled.
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