I have GTA 4 and when I got it I was running a E6320 @ 3.2Ghz with 2Gb RAM and 2x 8800GT OC in SLI (SLI isn't yet supported so for the purposes of this consider it as 1 8800GT). It ran ok, (1280x1024) but not all that great, quite choppy, so I bought a Q9550 which I clocked to 3.4Ghz and another 2Gb RAM due to everyone saying oh 'it needs a quad'. I now get 25-60fps (down to 25 when its raining, dark and driving quick) and it is pretty playable with med textures, 35 view distance, 100 detail, 50 traffic density and 16 shadows. However, my brother had a E6750 @ 3.4Ghz 2Gb RAM and a 1Gb HD4870. He got it the same time as me and he could run it same details as me with high textures (due to 1Gb Vram) but he was pretty choppy too. I got him another 2Gb RAM when I got my bits and his gameplay is pretty much as smooth as mine, and I have a quad! So I think if you already have 2Gb RAM a dual core which you can clock up to 3.4Ghz+ then don't go running out to buy a quad just yet, put another 2Gb RAM in and see how you fare.
So on the topic of a good gfx card, 512Mb Vram minimum, and something like a 8800GT (aka 9800GT) or HD4830 or better to be able to put the render quality and shadows up.
You really do need to be pairing it with a minimum of a 3Ghz Core 2, ideally a bit higher, and also 4Gb RAM will make a difference (in my experience) to how smooth it is.
Textures, Medium = 512Mb Vram, High = 1Gb Vram.
Render quality = graphics filtering which needs a decent card, only turn this up on more powerful GPUs, 8800GT / 4830 and up say.
View distance = Mainly Vram, you get higher with lower texture settings. With 512Mb and medium textures you will get up to about 40/100. Will also impact CPU and GPU.
Detail distance = not a huge performance hit either way.
Vehicle Density = Big CPU hit. Leave on auto value unless you have a tri/quad core CPU
Shadows = Video Card, turn up if you have the GPU power.