How would you get around speed-of-light delays? The speed of light is about 1 foot/nanosecond, so maintaining a 1-cycle access time @ 3 GHz gives you about 4 inches to play with. Current server designs have some headroom (100 cycles = 33 feet, while the actual physical distance is more like 1-2 feet), but as soon as you start going to another rack and want to maintain normal RAM access speeds you run up against physical limits pretty quickly.