What OS are you using?
What are the specs of your processor?
Best way to tell a bottleneck is to use the (old now) windows experience index (doesn't exist in windows 8+ i believe), but there are other ways to see bottlenecking. Windows experience index would rate all of your components on a scale, and it would show you which one is the worst. I forget some of the other rating programs, can anyone else point them to him?
If I had to guess just from looking at what you wrote, I'd say the processor. I think you can get away with 4 GB of RAM (with iRacing) and probably the GPU. Though adding 4gb more RAM would be the easiest and cheapest upgrade.