Cat 5e, Cat 6, GB Nic, GB Switches

We are moving. I know it sucks. We have the oportunity to wire the new building with Cat 6. I can also upgrade to GB switches and NIC’s in the servers. I have been doing some reading and am a little confused.

Cat 5e runs at 100 mhz. Cat 6 at200 mhz.
I have 100 mb NIC cards and switches.

How does a mb compare to a mhz?

If I go with GB NIC cards and switches is the cat 5e at 100 mhz retarding my speed?

If I go with GB NIC’s …. and Cat 6 at 200 mhz am I still retarding my speed?

Do I need special medium between a GB NIC and GB Switch or is cat 5e at 100 mhz way faster than GB’s/sec???

Explain this stuff to me? How does a Mhz relate to GB’s etc…………..

Thanks in advance.