Everyone has lag & stuttering when driving in GTA V. To be sure, more RAM is dfeinitely helpful, especially since you're running 64-bit Windows (required for GTA V).
The biggest issue is your CPU. It's a pseudo-quad-core (dual-core with HyperThreading), and its core speed isn't even as good as the minimum Core 2 Quad required for GTA V (Q6600 = 2.4GHz per core). Unfortunately, all of the i5 & i7 CPUs that would be compatible with your machine (Socket BGA 1168) are dual-core HT models, so even assuming you could replace the CPU you wouldn't see much improvement. And, given the type of CPU & your GPU, I'm betting this is a laptop, which means little to no capability to replace the CPU (let alone get a better motherboard).
Best bet is, if possible, add more RAM, & turn down the details & resolution to help keep it as smooth as possible. Also turn on vsync & set it for at least 30FPS.