See photo below.

I think one is just PCI.
Is not PCIe.
What is the other?

Here is the problem. I looked for video cards that work with the GX520 Dell.
Very hard to find a correct answer. Even the Dell site is hard to search for that kind of thing..
This is an example where Human Eyeballs are better that a computer database.

Please, what are they? 

[attachment deleted by admin to conserve space]It's a PCI slot. the added "Tab" is for use as "Riser" which ALLOWS the two slots to be put perpendicular to the board:

http://www.ebay.com/itm/DELL-OPTIPLEX-GX520-PCI-SLOT-RISER-CARD-ASSEMBLY-H5156-CN-0H5156-M5247-CN-0M5247-/391138254335?hash=item5b11a81dff

The reason for doing so is because the GX520 is a low-profile case. The riser allows installing full-height PCI cards.

I think this is the correct one.  Have attached a image and the link for the manual that I pulled the image from.

http://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-gx520_user%27s%20guide_en-us.pdf

Page # 124

[attachment deleted by admin to conserve space]Thank you both. I would never have guessed. It is a riser slot. 

You learn something every day!

I had been hoping twas another kind of PCI, like some forgotten PCI version.

The cost of the riser was rather high.

Here us riser:


[attachment deleted by admin to conserve space]Let's bring this post to a proper ending.
I want to feed two monitors from my old low profile Dell PC.

Look again at the picture above.
It seems there is,  somehow, a way to convert PCIe down to PCI.  Do you believe that?

Some pots on other forums STATE  the GX520 low profile desktop  has only PCI slots. Yet in one of the Dell manuals it was stated that the GX520 LP could have a PCIe device inside.

Well, to make a log story short, I bought a bargain dual port low profile  ATI dual port graphics card that is PCIe and not plain PCI. Can't useit. Unless I get an adapter.
OR
Maybe I could find a dual port LP graphics card tat is just PCI.

Yeah, If I wee to just upgrade my desktop to a modern model this would not be a problem. I refuse. It's the principal of the thing.

After going through all this torture, I may ha veto ask my doctor to put me back on Prozac.

So, I think this thread has ended. We are talking about the crazy stuff the PCI  industry did about 8 years sago. Namely, make a new hardware almost backward compatible;  then torture the end-user who needs it to be so.

References:
https://www.startech.com/Cards-Adapters/Slot-Extension/PCI-to-PCI-Express-Adapter-Card~PCI1PEX1
http://www.tomshardware.com/answers/id-1778463/pci-compatible-pcie.html
If you read the links above, have a box of tissue NEARBY. 
Good Bye PCI, I hardly knew 'ya.
It's a 13 year old computer, not worth upgrading.  Use it "as is" or give it away.
Early dual-core motherboard, Pentium-D.
Just barely Win10 compatible.
Computer_Commando, you are right. And I should know better. 
Let this be advice to anybody out there wanting to use old computers. It is hard to maintain old stuff and compatibility issues become overwhelming.

I have two or three possible solutions. I now am waiting some interface cards to see if I can upgrade this old clunker. Years ago I did dual monitors with very old equipment and I think it still is within pragmatically.  Maybe in a week I will post results of my quest. Our I will switch to a newer computer.Update. Here is a much better photo of my GX520 slots.

You can see that one slot for sure is PCI.
The other is, I think, a slot for the Dell riser assembly for this model. Note that this is a smaller than normal tower and takes only half high cards. Also, it looks like  one slot is missing the connector. As if they intended to include it on a different model  Maye this same motherboard was used on the full high tower.

I have ordered a riser card cage that claims to be for this model.
End of story. No way. Dell does not mke a riser that converts PCI to PCI E. 

So, I have another desktop with dual video outputs.But no slots. It is a Optiplex 745 ultra small form.

That DVI does both VGA and digital output on two channels.
Here it is driving two TV sets.

So, this topic can be closed. 
Quote from: Geek-9pm on September 01, 2016, 11:41:04 PM

Dell does not mke a riser that converts PCI to PCI E.