Why it matters is if this is a 32bit OS my comment won't apply. That 4GB is now slim. I'd get 8GB today. We're also pushing the CPU a bit hard but the low RAM could be taking its toll.
As to GTA5, I'd reduce effects to carry on for now. I would not change a thing besides RAM if the OS is 64 bit.
I have older gaming PC running all but GTA5 well (i.e. FarCry 4 runs smooth, but GTA5 drops below 30fps):
GFX: Sapphire HD 7850 2GB OC GDDR5
MoBo: Gigabyte GA-965P-DS3 - Intel P965
CPU: Intel Core2 Quad Q6600 @ 2.40GHz
RAM: 4x 1GB GB ADATA + Series DDR2 800
Not ready for upgrade all scenario, looking for minimal investment gaining most performance.
Q1: Is there a point in upgrade GFX at all? (MoBo does not suport PCIe 3.0)
Q2: Or do I better go i7 way and change CPU & MoBo?
Q3: If upgrade CPU, most likely need to upgrade RAM too? Reuse would drag the i7 down I suppose?
Many thanks for good advice!