Asterisk implementation

Hi i am new in Asterisk …

I want to do the following set up … so pls guide me the right way …

Connection :
PC1 <----> cisco Switch <—> cisco Router <---->cisco Switch <—> PC2

i wish to install Softphone in both PCs [ planed to install cisco softphone]

so if i want to make call from PC1 to PC2 … Really do i need asterisk …? or any other software is enough …? if so, pls specify …

Because in the above scenario i wish to implement QOS in CISCO switches and routers and i want to monitor the traffic …

pls help me

thanks in advance