Cat5 supports 10/100 Ethernet and a maximum of 100Mbps and no greater. Because of the potential for crosstalk, interference, and other problems, it is very rare that Cat5 cables actually achieve 100Mbps. This is around 12.5-12.8MB/s maximum sustainable throughput.
Cat5E attempts to achieve 1000Mbps (or 1Gbps throughput). This was an important milestone, because this translates into roughly 125-128MB/s, which is close to the read speed of a single hard drive.
Cat6 achieves over 1Gbps up to a potential 10Gbps. You will see this featured with "Gigabit routers" and "Gigabit cables". One gigabit is 128MB/s. Reaching 10Gbps would give you a transfer rate close to 1280MB/s or 1.25-1.28GB. This is known as 10GBBASE-T. Most home and SMB routers today are sold with gigabit networking capabilities.
To run at the speed of the cabling, you require every component to match the maximum sustainable throughput. This is important. If you want to run Cat6, you need:
Computer network adapter card that is gigabit (Cat6) compatible.
A router that is gigabit compatible.
Category 6 wiring.
Cat6 is better if you are worried about interference. While Cat5e attempts to reach the 1Gigabit milestone, Cat6 is actually certified to do so and can very well exceed it, approaching 8-10Gbps on many internal networks that use certified Cisco-Linksys equipment.
Find this information useful? Let me know. The most important part to remember is that the cabling gives you access to faster transfer rates. That means you need to have matching equipment on every level. While backwards compatible, it would make no sense to slow down the whole network by using a hybrid network using these types of cables. For a more detailed explanation of the matter check out Gigabit Ethernet - Wikipedia, the free encyclopedia