I have GTA IV and without an ENB, can run pretty well max settings (Except shadows must be High, not Very High) and with some ENB (Can't remember what it's called) I can still run with those same settings. I have found a different ENB however called CryENB 2.0 which I love. It doesn't have the red sky bug with 18.104.22.168 and looks great, but it is pretty demanding and lowers my FPS a lot.
I've heard that since GTA IV was a sh*tty PC port, it depends mostly on the CPU rather then the GPU so I should get a better processor if I want to run the game good. Then I also heard that the ENB mods utilize the Graphics card a lot better so the processor won't matter if I am aiming at using an ENB.
Also I want to run the game with some modded cars so I'd imagine that would require a lot of graphics memory.
Currently I have...
CPU: Intel i3-3220 @ 3.30 GHz
Graphics Card: NVIDIA GeForce GT 640 (Aprox. 4038 MB)
OS: Windows 7 64 bit
So, my question is what should I focus on upgrading first? The graphics card or processor?