Quite frankly, using a DSLR for video, at least for the reason MOST still shooters want to use video, is a BAD idea. These cameras are simply NOT set up to be ad-hoc video cameras. They are terrible at it.
If you just want some quickie video of the kids or something similar, but a video camera for a few hundred bucks. It'll save you a lot of time and headache. If you want to learn to make films, DSLRs are the most cost effective way to get into that, but be forewarned that movie making budgets are an order of magnitude larger than doing similar still shoots.
As for the computer requirements, you will be asking your computer to read/convert 24/25/30 2MP images per second. Any modern computer (as in the past couple of years) is up to the task with proper software. Avatar was edited on dual core PCs from 2007.