I am not having any luck with those part numbers you provided.  517123-001 appears to be a HP ATI Radeon graphics card and AriesATX-21-M06 appears to be another old defunct video card.  I have searched HP's website and Google and those numbers seem to get me nowhere.  Are there any other model numbers on the outside of the computer case?  Usually the model number will be on a sticker on the back or bottom of the computer.
 
"Usually" the problem you are reporting is caused by an incompatibility with the motherboard because the motherboard doesn't support UEFI firmware and instead uses the old BIOS firmware.  Make sure that your motherboard has the newest firmware available on HP's website.  Some motherboards simply will not support any video cards newer than the 500 series NVIDIA video cards and a majority of 600 series NVIDIA video cards.  It may be the truth that the GTX 950 is too new for your motherboard and will never work.

I don't see any BIOS version updates available on their website.
http://support.hp.com/us-en/drivers/selfservice/HP-Pavilion-Elite-e9200-Desktop-PC-series/3999455/model/4007332
 
Regarding UEFI support and graphics card upgrade issues:
http://h30434.www3.hp.com/t5/Desktop-Operating-Systems-and-Recovery/uefi-bios/td-p/5507082
 
If you want to use a NVIDIA graphics card, you will need to purchase one which is from the 600 series or older which is not UEFI compatible.  You also have to consider that your power supply is only 300 Watts.
 
This is about the highest performance card I could think of which will work in your system (even though it does supposedly [doubtfully] draw a little too much power for your power supply):
EVGA GeForce GT 640 Superclocked
http://www.evga.com/Products/Product.aspx?pn=01G-P3-2642-KR
It is a very low performance video card though.  I don't think you would be very happy with it.
 
If you want to push the abilities of your power supply a little further, you may consider the GTX 650 or GTX 650 Ti.  NVIDIA recommends a 400 watt power supply for the GTX 650 or GTX 650Ti, but it will probably be fine since numbers from people online show that their total system power usually maxes out between 200 and 250 watts with the GTX 650Ti.  The GTX 650Ti is substantially better than the GT 640 and is about the best NVIDIA card you could hope to use in your system.
https://www.amazon.com/gp/offer-listing/B009KUT3D6/ref=dp_olp_0?ie=UTF8&condition=all
 
If you do decide to purchase a 650Ti, I would not recommend that you buy a factory overclocked model and would not recommend overclocking it manually either.  The power consumption margin is close enough as it is at stock factory NVIDIA reference speeds.