How tactical do you want to go? What size do you require? Do you want to use belt order or webbing as well? Carry Military Radio? Expandability using other pouches (PALS Molle or PLCE)
The Karrimor Tecmac 50/Predator 45 is quite a useful Tactical Backpack, it works very well with a PLCE webbing set for better weight distribution for heavy load outs and with twin 15 litre PLCE side pouches and Karrimor Yoke you could get a very flexible system.
http://www.youtube.com/watch?v=qQmgIYh8vOYhttp://www.youtube.com/watch?v=-moeHA9chaIIf you need to care a huge load then the Karrimor Sabre 80-130 litre would be a good starting point.
http://www.youtube.com/watch?v=UkrPiKwYiGQ