it depends on how detailt the map should be! You should use the LOT system to save memory on a large map and you can get a few FPS more out of it by choosing an camera angle that point a littel bit down, so that u can see whats in front of u for about 100 meters or so. With that way (wich is also used in GTA)you see enouge in front of u but not too much on the horizon, what would cost FPS an memory.
If you wanna make a game like GTA i would say, make it in blocks with good textures. (a singel large block, with a good texture, can look like a high detail Sky scraper)