preventing usb bus overload.

I would only think that would be an issue if both devices are drawing their power through USB. Even at that, I am not so sure. A good quality motherboard shouldn't have any power issues.

Firewire doesn't engage main computer processor in transfers. It might be irrelevant if you don't do any heavy processing at the same time. There are two different types of USB DACs Synchronous and Asynchronous. Synchronous DAC timing is created (synchronized) by computer's USB bus and traffic on this bus (or computer's activity) can possibly affect timing (jitter). Jitter converts to noise. It is very crude scheme. I suspect that most of new DACs use asynchronous transfer in which DAC is running with its own stable clock synchronizing with computer (to avoid loosing samples) by storing samples in the buffer and telling computer to increase or decrease number of samples per frame. When buffer is too low it sends message to computer "more samples per frame" until buffer is close to be full, then it sends message "less samples per frame".
In this scheme timing of computer is irrelevant, other than overall electrical noise it is creating. Hard disk timing is irrelevant since it contains data (and not the music), meaning - it has no time base.

I have operated two Windows 7 based systems with USB HD's and simultaneous USB feed to asynchronous DAC's with no obvious problems, and no sonic compromises that I am aware of. Of course, YMMV, depending upon a whole lot of variables in computer audio...

You can connect an external hard drive using esata, and use usb for the dac. Also, don't overlook the power supply. Its very important to get a good one.