Some light reading:
http://en.wikipedia.org/wiki/Twisted_pair
http://en.wikipedia.org/wiki/Category_5_cable
http://ezinearticles.com/?Cat-5,-5E,-6,-and-6A-Cables---Distance-and-Speed-Limitations&id=1237951
I believe the actual xfer speeds and times can be affected by using a mixed of different cables however connected speeds shouldn't be. Realistically, people would run all the same type of CAT level. I have a mixe here however thats off the switch and not in between the switch and router and modem. Thats all CAT 6.