Do I need a firewall?

Any type of firewall is a good idea, especially if your going to be using a broadband connection. Software firewalls always tend to run a little slow from my experiences with them, since they are running on top of other programs, and have to use your computers resources to filter all the incoming packets. I'm using a cable modem, and without the firewall active, I can acheive speeds of over 100KB/sec constantly. With software firewalls running, I'm lucky to be able to get 20KB/sec from the slower filtering process. A hardware firewall would always be the best solution, as they are usually even more configurable than their software counterparts. They are always much faster as well, you should see very little to no speed reductions. Heres a webpage that should help you further research software vs hardware firewalls: http://www.firewallguide.com/
Hope this helps.

It all depends on what you need. If you will use the internet (not ISA) for browsing purposes only, email only, you will survive without a firewall.
In the case that yoy setup a WEB server sitting inside of your LAN, yes you will need a firewall, since internet people will be "close to" your lan.

ISA is a pretty good firewall in itself. This is not a next version of Proxy Server, this is a real firewall. I ran the trial version and was very impressed tight control it placed on my internet point of presence. It literally closed all the ports. I had to configure the ports I wanted to open, how my internal network was going to communicate, who need what services. I did not have to install special software on each client (as with Proxy server). The axiom to remember is only open the ports you need (less is better) and then only open the ports you need for the machines you need (a subset of less is even better). To answer your question of do you need another firewall whether it be hardware or software, no. ISA is a good firewall.

Let me know if you have any questions.

If the Question is do you need a firewall if you have ISA? No, ISA is a firewall, and a good firewall. If the question is, is ISA the best firewall out there that you can get? the answer is No absolutely not.
It all depends on your security requirements. Is it a cost effective solution for many businesses - yes. Again, it all goes to your specific business needs.

Get another firewall!

Stop the insanity of believing MS products are secure. They are NOT, nor will they ever be secure.

Case in point MS' very secure IIS security was shredded by numerous exploits such as Nimda and CodeRed.

ISA serve willstop the newbies, the wannabees and the half hearted, casual glancers, (assuming that YOU secure it properly)but it is not going to prevent someone with enough determination (locks were only invented to keep the HONEST people out).