Fast and cheap data aquisition
When you need to trigger off of a fast signal, you don't need an ADC. You can just as easily use an Analog Comparator. Your ATMega already has one and I believe it will be fast enough. I believe the 328P doesn't have an internal reference, so you may have to provide one (could be just a voltage divider).

Then you can have an interrupt fired when you cross the threshold voltage, and you're set.