EDIT: OK, I have found out my amp is 8 ohms. SOOOO this means I can get an 8 ohm speaker. Can I use any speaker that is over 8 ohms, such as 16 ohms? and would it damage my amp if I use a 4 ohm speaker?
Also now I have a problem with wattage.....

can I use a 200w or 400w speaker in a 100w amp? would this affect performance in any way? also is it unwise to fit a 50w speaker to a 100w amp?