Complex sentence: Depending on the pixel resolution per square meter, covering a large area could require several gigabytes of memory, while an average video card has only 1 gigabyte available, not including what is used by the rest of the game.
Split into simpler sentences: Depending on the pixel resolution per square meter, covering a large area could require several gigabytes of memory. However, an average video card has only 1 gigabyte of RAM available, not including what is used by the rest of the game.