You actually don't have a dedicated graphics card on that unit, Intel HD Graphics 4400 are the built-in graphics that come supported by your CPU.
It is always touch and go to recommend a dedicated GPU for a pre-built workstation like the one you have because the motherboard, internal space, and power supply could all be very serious issues. If the draw from the new GPU to too much for the PSU you could short the entire rig. If you don't have enough space for a dedicated GPU that is obviously also a problem.
The HP Pavilion 500-281has a 300W power supply, and I can assure you it isn't built to provide reliable power for even a halfway decent GPU. If you MUST try to upgrade your rig either of these cards would be by far and away your best bet, but please be aware that if you try to install these you will be running a very serious risk of damaging or even destroying your rig:
Zotac GTX 750ti
PNY GTX 750
Neither of these cards require extra connections to the power supply, BUT they still both recommend having at least a reliable 400W PSU in the system.
If it were me, I'd honestly just save the money to upgrade to a new desktop down the road that has the specs I need. You don't need to break the bank to find modern pre-builts with 3 screen support.