I'm planning to change the network backbone to gigabit LAN. The cabling to the floor boxes for the office is all done in CAT5 currently running at 100Mbps - is CAT5 cabling Ok for gigabit LAN?

Probably. Some cables might have excessive crosstalk or return loss which will not work well at gigabit speeds. If you have problems the cables can be tested. If running new wire use CAT5E or CAT6.

Thanks for the info.

When using a cbale tester what sort of results should I be looking for? Max/min attenuation limits?

This might help:
http://www.proav.de/index.html?http&&&www....ables/CAT5.html

Very good link - has all the info I could wish for - thanks.