For the amount of data you'll be pushing, I'd highly recommend getting something like the Cisco ASA 5520. I've used these in the past, and they make a very capable firewall. They can also provide VPN services if you desire. The spec says these can handle up to 450Mbit and 280k sessions.
If you do end up going for a software-based firewall (which is somewhat of a misnomer, as all firewalls are software-based. I digress), I'd highly recommend that you go with something like PFsense. It's BSD-based, so the network and firewall performance and security is as good as it gets for this type of product. I shudder at the thought of using ISA Server to route and inspect 400Mbits...
Edit, after clarification of question
No generic comparison can be made between a server-based solution and an appliance. There are vast differences in the quality of hardware available for each category. You'll find some "appliances" that are just commodity hardware, cheap network cards, running linux with a nice GUI frontend. Put that appliance up against a quad-core server class-machine with lots of RAM, a couple PCIe cards, and PFsense, there's no question that the server will walk all over the "appliance". Conversely, there are some appliances that employ real hardware ASICs for processing packets. Depending on your packet load and the type of filtering you want to do, these can be very high-performance.
In the end I think you're either going to have to try out some of these solutions to see which will work better for you or possibly contact the vendors directly about specific models.
Like I said, I have a fair amount of experience with both the ASA platform and PFSense. Both platforms have strengths and weaknesses, but in your case, I'd imagine that either would be able to deal just fine with the loads you're expecting.