To monitor traffic you will need additional resources. Many cisco device have Netflow install (Pix 525 is not listed) But another device in your network may be capable and can be used to estimate traffic
The solarwinds Real-time NetFlow Analyzer is a free tool that can be used to see the the data on the network, but it is looking a the Netflow configured on other network devices It will also show utilization. I am not aware that PIX has a means of monitoring.
I did find some interesting tools on www.paessler.com. Some are free and some cost $$$.