If the PC is running slow it is highly unlikely that the graphics card is the culprit and I doubt very much if you will see any improvement by replacing it. The reason for slow running is much more likely to be a software issue, probably due to accumulated "junkware" and "bloatware" that gets accumulated over the years and loads lots of unnecessary stuff that runs in the background and slows the computer down. Probably the best thing that you could do would be to back up your data, do a clean install of the operating system and re-install any software that you need. This would be much more likely to improve things than replacing a graphics card.