I have similar pc, even weaker: 550 ti 1gb, core 2 quad Q8200, 4 gig ddr2 ram, windows 10 64-bit. I run the game with 1280x960 resolution, max shadows, race time scale on, maybe few things on high or medium. Most important settings are in my nVidia profile, I will list the things I have changed from default:
Antialiasing mode - override any application setting
Antialiasing - 8x cssa
Maximum prerendered frames - 1
Power management mode - max performance
Texture filtering anisotrophic sample optimization - on
Texture filtering - High performance
Vertical sync - Adaptive (half refresh rate)
That last line, on my 75 hrz monitor, gives me constant 38 fps. Now, the game runs at 100-130 fps without AI, but in G-Motor games I found that there is very noticeable and unpleasant stuttering when frames drop or rise, form 100 to 80, from 70 to 90. On certain tracks that happens on important corners and feels awful. Why half refresh rate, and not full? Well, I can't hold solid 75 behind full field of cars with max shadows, and 38 is very close to the GPL's 36 fps I was so used to, and will not drop lower no matter how much smoke or cars are on screen.
So this is my solution... if you want, you can drop shadows to medium and try just normal adaptive vsync. Don't use the game's vsync tho. You should get it to run pretty smooth without turning off race time scale (I kinda like running laps while the sun rises
)