List the full specs of your system, for better suggestions.
The motherboard may have a PCI-E slot for video, which is current.
Or it may be AGP type, still in use.
Then there are those micro-ATX designs that have no provision for an add-on video card.
The power supply in the system may not be adequate to support a power-hungry video card.
Also, what intents do you wish to put the card to use with? For gaming, you would want fluid video, with the most realistic detail. If the CPU isn't up to the task of 'pumping' data rapidly enough, then you would be disappointed.
90% of systems can take upgrades.. but those that cannot are not worth the cost of having been purchased originally, unless intended only for Web browsing or Emails.
F@H.. to solve mankind's maladies.. in our lifetimes!