Ook with only 1 case fan, your temps are expected to be very high. Most graphic cards nowadays blow all their hot air directly into the case. If you're not doing much cool air intake and taking away that hot air, it's going to sit in your case and your temps are going to shoot up very quickly. Here is a nice video that explains temps you can expect with only one fan and where that fan is positioned, as well as effect of adding more fans:
https://www.youtube.com/watch?v=8OmkmluAYAQ
Also, gpu with only one fan is going to run hot if you're playing games. The reason why most gpus nowadays have 2 or 3 fans on them is to improve cooling and therefore performance. Higher end cards do run hot when gaming, and that hot air has to be dissipated. On a lower end card with only one fan, performance is limited, and due to lower cooling, you can expect high temps as well. You should compensate here with good case cooling to help it out.