Hi,
The IOS FW datasheet has performance numbers for different ISR platforms.
Also, in ISO FW Q&A:
Q. How much memory does Cisco IOS Firewall use?
A. Cisco IOS Firewall consumes roughly 700 bytes per connection for basic inspection. More detailed application inspection will consume more memory: for example, FTP, HTTP and VoIP AIC.
Hope this will give you some ideas.
Alex Yeung