That's better than the difference between Pitcairn and Tahiti.
7850: 32 ROP, 1024 Shader, 2GB VRAM, 256-bit bus
7870: 32 ROP, 1280 Shader, 2GB VRAM, 256-bit bus
7950: 32 ROP, 1792 Shader, 3GB VRAM, 384-bit bus
7970: 32 ROP, 2048 Shader, 3GB VRAM, 384-bit bus
The 7970 has 60% more shaders, a 50% higher memory bus, and 50% more VRAM, and is ~20% faster. Nvidia getting a 25% improvement over 33% more shaders and a wider memory bus is pretty impressive. If they priced this at $599, or $550 ad knocked everything down $50, they would crush AMD if they still refused to lower their prices at that point.