The 3650 is faster than a 6800 and even with your CPU limiting it a little it will still be better than a 6800 in your system. Of course the Radeon 4670 is better, much better, but for that I would recommend you replace your PSU and upgrade your CPU to get the most out of it. As for the 400W unit recommendation that's just to be on the safe side. Even with a Radeon 4670 your total power draw should be a lot less than 200W when playing a game, but if your PSU is really cheap then that may be more than it can handle despite being rated at 305W.
The radeon 4670 is rated to use like 60W (at least according to the launch documents). The 3650 uses even less than that. Since those dell 8300s use P4 CPUs, not even dual cores at that, the Radeon 4670 would be so limited that it would probably preform the same for you as a cheaper 3650.