ram differences?

[andyjamesnelson]

hey just looking at crucials online store and i'm trying to work out what the difference is between the ram they stock?
i'm looking at DDR PC2700 modules, there are 3 listed:

512MB
£53.99
£63.44 CT6464Z335
DDR PC2700 • CL=2.5 • Unbuffered • Non-parity • 6ns • 2.5V • 64Meg x 64_

512MB
£71.99
£84.59
CT6472Z335
DDR PC2700 • CL=2.5 • Unbuffered • ECC • 6ns • 2.5V • 64Meg x 72_

512MB
£109.99
£129.24
CT6472Y335
DDR PC2700 • CL=2.5 • Registered • ECC • 60ns • 2.5V • 64Meg x 72_

just wondering what makes up for the price difference? there all 512mb but have a real difference in price!