What is the difference between regular Cat 5e and Cat 5e 350MHz?

Contrary to popular opinion and cable marketing hype, a MHz rating is not a measure of speed. When cable is tested, a series of signals are sent from one end of the cable to the other. At the other end, a receiver measures the signal strength and integrity to make sure that the cable meets its standard, in this case the Cat 5e standard. The MHz rating is just the range of frequency used to test the cable. The Cat 5e standard calls for testing over a frequency range of 100MHz.

Both regular Cat 5e and Cat 5e 350 MHz are suitable for gigabit networks.
Theoretically if one could utilize a greater range of frequency for transmission, one could send more data across the cable and improve speed. However, at this point in time, almost all computers and networking equipment only have the ability to transmit across a range of 100 MHz, so the hold up is the electronic equipment, not the cable. Plus, just because a box of Cat 5e cable hasn't been tested to 350MHz (or 400MHz or 600MHz or what have you) doesn't mean that it isn't capable of carrying a signal across this range.

When people call FOURPAIR.com and ask whether they should buy regular Cat 5e or the more expensive Cat 5e 350MHz, we recommend the regular. When you pay for a higher MHz rating, you're basically paying for extra testing. That testing may or may not be worth the extra cost to you. It may be worth it if future-proofing is a major concern that will keep you awake at night, but you have a tight budget to work with at the same time. If future-proofing is your number one concern and your budget is looser, skip the high MHz Cat 5e and go for the Cat 6 which is significantly different from Cat 5e cable.