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What's the difference between Cat5, Cat5e, Cat6? Which one do I need?

Here's various bits of info:

Notice the difference between MHz and MBps? Cat 5e and above also break away from the MHZ=Mbps formula.

Now consider the 1000Base-T Gigabit Ethernet, which always energizes all four pairs at once! The bi-directional dual duplex transmission scheme employed by 1000Base-T actually requires each end of a channel to transmit on one conductor of each of the four pairs simultaneously.
Not a good idea on Cat5 - because of cross-talk and return-loss.

Category 7 cables will be "fully shielded", with individually screened twisted-pairs and an overall shield.

Category 6/class E delivers the highest level of transmission performance available without individually screened pairs. For the vast majority of business and institutional applications, 250 MHz bandwidth is more than adequate for the life of the cabling system, making category 6/class E the perfect choice for generic premises cabling.

The difference between Standard Category 5 (Cat 5) cable and Enhanced Category 5 (Cat 5e) cable is that Cat 5e is what we call a "PowerSum" rated cable. A Cat 5e cabling system provides additional performance margins to help ensure your cabling infrastructure meets the demands of the future applications that are emerging today such as 622-Mbps ATM and Gigabit Ethernet. What this means is that multi-pair protocols that are emerging now, such as Gigabit Ethernet and 622 ATM, will operate on Cat 5e cabling infrastructures where as we can only say that these may operate on standard Cat 5 infrastructures (this issue is still being debated). For these new protocols to operate on a standard Category 5 platform each circuit would have to be independently verified and no guarantees could be given.

Note: STP = Shielded Twisted Pair, and ScTP = Screened Twisted Pair

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