scope with 18f2550
They are likely implementing an external ADC. Offhand I don't think the 18F2550's manage even 100K samples. You can speed it up by reducing the resolution of the samples, but not anywhere close to 800K samples.
We use the 18F2550 in our USB Semiconductor Curve Tracer... a very nice PIC:
**broken link removed**
Added after 11 minutes:
I looked at the link and think the claims are not good. For instance, they mention shared sampling rate. That's not possible with a single 18F2550 since the ADC's are multiplexed and each must timeslice and take a turn. I don't understand how 80KHz turns into 800K samples, and so on.