First as a Bank they are not in compliance with PCI requirements if they do not have an IDS system. They really need to have an IDS.
As for the first part of your question, can a well configured firewall act as an IDS, it depends on the Firewall and your IDS needs. Most Firewalls include some IDS signatures, like the PIX. Checkpoint includes thousands, and you can set triggers and alarms to act as an IDS.
Personally I prefer a good Protocol Analyzer and a Laptop.
I use a an older HP laptop with a promiscuious NIC, and Etherpeek NX 2.0 and a SPAN port on our CAT5000. I build custom signatures to filter out whatever traffic I want to see.
You would be surprised at how well it can work.
But for a Bank, I would use a COTS tool, like Blink, or Enterasys Dragon.