How can I monitor my WLAN data usage ? is there any firmware router update that allows this feature?

You can use Radius Server for a granular control over your WLAN and get all type of Statistics you would ever need.

Will go through this, thnx alot

You can view your traffic graph by MRTG server or some other traffic monitoring server/system.Apart from MRTG some other users prefer Cacti.This graph can be sourced from different points.This graph can be taken from the Access point that you are connected to or even can be taken from your Wireless LAN card adapters.

However, to acquire a graph from specific interface/adapter you will need to run SNMP protocol.If the device doesn't support SNMP then you can't get graph.Some other devices like Mikrotik shows real time usage of individual client.

It depends on your topology and the types of device you are using if that supports.