If you capture via firewire, you are stuck with HDV. However, I have done this and what works best is to capture in HDV. Then create a new project in Standard Def (wide screen, though I guess you could do full screen) and then import that captured HDV footage into that project and then cut it into the timeline. You do have to resize the footage, but it’s not as tedious as it sounds PLUS you have the advantage of being able to reframe it (almost as if you shot 2 cameras!) Depending on your settings and system, you should be able to play the HDV footage unrendered in the timeline (set quality to automatic)and render it once at the end. I did this in 1080i60, but the same should apply to 720p24.
The other option is edit in 720p24 and export a standard def final when you are done. You don’t need a substantially faster system to work in HDV then DV (they have the same data rate.)