Data coming out of cache won't saturate 6Gb/s, but will be faster than 3Gb/s. I'd say there will definitely be some real world improvement, especially when we see 750GB and 1TB per platter in the near future.
That recommendation only applies if the system is using the swap file, and even then the performance impact is negligible. Most common wisdom about mechanical disks falls apart in the presence of high memory capacity.
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750MB/s sustained transfer rate?
Partitions?