Not sure about some of the DOS tools out there, I guess they do exist, but I've put firewall CPU's under stress by sending lots of ICMP packets with a large buffer size, I was doing it on a course the other week. Then used Cisco Pix Device Manager to watch the CPU on this particular firewall go through the roof.

So you could have a batch file with 'ping 192.168.0.10 -l 65500' and just keep on double clicking it, this will eventually put a test host on a test network under some stress.

James.