Just use cat5e and 80m should be no problem.  Cat5e is much better than Cat5 because it'll help you get to Gig without any issues, in case you need to.

Mike .... cat5 or cat5e are unable to handle gigabit transfers. You need cat6 or 7 for that.

To answer the other half of your question if you want to cable longer than 100m with cat-5 then you will need a hub/switch at the end of that 100m segment to prevent data loss.  The hub/switch refreshes the strength of the data transfer allowing it to be transferred another 100m without data loss.  If you are going a fairly long way though you might consider fiber because its faster and can have longer distances, but I don't remember what they are off hand.

Wyliecoyoteuk ... you said : << In a 100Mbit network, the maximum span (distance between any 2 nodes) is 200Metres. >> 
Well ... that is wrong. The maximum span depends mainly on the type of cable one is using and then on  external perturbations and the output power of the emitters.

kronostm:
It is nothing to do with power (although that can have an effect also), is to do with timing.
to be precise, 205 metres is the maximum distance between any 2 nodes on a 100baseT network with Cat5 cabling).
Over this distance, you get slowly increasing data collisions, due to overexpanding the "collision domain"

It is to do with CSMA/CD signal timing, not signal strength.
In simple terms, the time a node waits before assuming  no collision, and sending the next packet is the time it takes for a square wave to travel 205Metres.

http://support.3com.com/infodeli/tools/nic/3c905/docs/obs/09_apxa.htm
http://www.geocities.com/SiliconValley/Pines/1572/csmacd.htm

And yes, I know that you can exceed this and get away with it, and number of nodes, number of repeaters, quality of cabling and hardware will also have an effect.
The more you load a network, the more important these factors become.
But exceeding the limits usually causes problems which are more noticeable at times of high network traffic, and can cause collision storms and data loss.

If you stick to the 100baseT rules, 100 metres is maximum cable run, 205 metres is maximum network span between nodes, maximum 2 hubs (stacks and switches are a different matter) less than 10 metres apart.