Your mobo should be able to support quad core. Go to mobo manuf website and look for CPU support and check to see if you need to update bios also for quad core. Here is a link to ebay to get cheap quad core.
what's the best GPU i can get for Core 2 Duo E8500?
If you want to game with that cpu you are looking at games from 2013 or before. After that it's simply getting to weak.
However list your full specs and psu in detail so I can recommend you something for little money since that system is already 10 years old and it's time to move on.
Your mobo should be able to support quad core. Go to mobo manuf website and look for CPU support and check to see if you need to update bios also for quad core. Here is a link to ebay to get cheap quad core.