There's not really a mini-PIX, but all IOS routers, including the 827, can run a feature set called IOS FW, which basically provides additional firewall features on your router, effectively turning it into a stateful firewall similar to a PIX.
You'll need to purchase the additional feature set, but just ask your reseller for the firewall feature set and you'll be off and running.
The docs are here: http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t5/iosfw2/index.htm, the main one you'll want to look at is the Context-Based Access Control (CBAC).
