You should probably contact an NI application engineer for help in determining the proper hardware. There are a lot more requirements that need to be considered other than just the number of inputs (i.e. sampling rates, signal conditioning requirements, etc). If USB is the route you want to go, I would venture to say that a cDAQ chassis would probably be the better choice since you can mix and match modules. Of course, this all depends on your budget limitations.
Don't quote me on this because I've never used it before and I'm not familiar with it at all. But I think the Sound and Vibration toolkit would be an extremely helpful tool for analyzing/plotting accelerometer data. My company recently considered building a balancing machine with accelerometer outputs and were told it would be a necessary toolkit.
