for gfx cards the gpu comes first, then the memory. for instance, a radeon hd 6950 with 1 gb gddr5 ram would be much faster than a radeon hd 6670 with 2 gb ddr3 ram. the 6670 has a slower gpu that cannot utilize the 2 gb vram it's equipped with. moreover, the 6670 has slower ddr3 memory while the 6950 has faster gddr5 memory and gpu that can efficiently use all of its vram.
multiple factors decide which gfx cards will be faster. ram is only one of those factors. other factors are gpu clock speeds, memory bus width, memory bandwidth, architecture/fabrication process etc.