Performance is a multi-way dance.
First, the amount of RAM is important. If you don't have enough, your disk gets involved because you have to go "virtual" on any large game. Setting the video to higher density uses more RAM. You need big enough RAM or small enough image density settings to not do that. Once you reach that point that you are no longer swapping, you're done. More RAM won't help. You can find when you are at that point when Performance Monitor says you aren't swapping or paging (using virtual memory) any more.
Second, your CPU will make a difference. I don't know what GTA uses but if the game is not designed for multiple threads then the only thing that makes a difference (for the CPU) is raw CPU speed. Believe it or not, if you have a 3.2 GHz i5 and a 3.2 GHz i7, it is possible that there will be no difference between the two in terms of performance - if the game is single-threaded.
Third, your graphics card off-loads the CPU to a VERY large degree, so the polygon rate of your graphics card will make a world of difference. The other settings, because they are interpreted by the parallel processor in the card's Graphics Processor Unit (GPU), take extra computations the more you adjust them away from whatever is normal for that game and that card.
Fourth, it is unlikely to be an issue, but SOME displays are slower than others when you "paint" them and that hits the frame rate hard. If the screen can't keep up with the card, the I/O manager "stalls" things and you lose frame rate.
Fifth, some games either have a huge map area to load or they "fragment" the maps and load them only when you move to them. If you have a slow disk bus or a slow disk, you'll see a "loading bump" when you cross into a new area. If that happens, you have very little you can do. Using a solid-state disk usually makes this bump small, but SSDs have limited size and you can't load EVERY game there.
It's all a big balancing act, but that is why gaming machines are so expensive. You need a lot of fast RAM, a really fast CPU, a fast disk bus, a fast disk, a fast GPU, and a fast display. Fast does not equate to cheap.