Which is better for cooling down my computer system- a graphics card a heatsink or a fan?

Generally the higher end video cards will have both a large heatsink and a fan. Lower end video cards will commonly only have a heatsink as they use less power and thus do not get as hot. If you are having issues with your computer overheating then you may need to look into improving the air flow inside your system. You can do thise by tying together cables and positioning them in a way that they don't restrict air getting to your CPU and GPU. You may also want to install a case fan or two if you do not already have one to help circulate the air through your system. If you're looking to upgrade your video card I would worry more about performance, price, and compatibility than I would whether the card has a fan or not. Everything else being equal though I would personally feel more comfortable if my video card had a fan in addition to a quality heatsink. Also take note of the customer reviews on any video card before you buy it and check to see if anyone complains about cooling issues with the card. I hope this helps, thank you for using IMshopping.