I am having a wireless solution at my work in which i installed 50 pieces of rb751. Also i have a RB1100AH. i want to monitor the wireless routers. what is better to install the dude on the RB1100AH and connect to it through the PC and scan the network. or just install the Dude on the pc and scan the network ??

There is no Dude package for RB1100, so you don't have this choice anyway

okk but what is the difference if i want to install it on a separate rb751

RB751 is not powerful enough for the Dude package. This package needs a lot of space and more RAM.

Okk i understand but i mean what is the difference between dude if installed on pc only and dude if installed on routerboard??

The Dude package on RouterBOARD is to be used as an agent. This gives your Windows PC Dude the ability to monitor devices inside some LAN. See here: http://wiki.mikrotik.com/wiki/Manual:The_Dude/Agents

And what about installing the Dude on x86 on routeros? Is it good idea to do this?