I don't have a frequency counter, and I'm unable to derive the maximum frequency of the shiftOut clock based on a default arduino.

Long version of question short:  How scalable is an LED display with the piles of shift registers I'm planning on using, while maintaining a reasonable refresh rate.

I know if I shift a byte as quickly as possible the 8 leds are a dim blur; but if I want to start making a display of say...256 or 1024 leds;

what kind of refresh rate can I expect assuming mild/moderate other calculations such as serial io (to get the shift data) or parsing tables in ram...

the de-facto si-po shift register is the 74hc(t )595
the (t ) is the difference between cmos and ttl level; double check for what you're using.

There's also the chips from Maxim (max) which are more expensive but have a lot of integrated features.

As for reading buttons; I can't recommend a parallel in-serial out.

I would like to interface with a relay board - probably dead-bug style - to switch line current to many several channels of animated christmas lights.

I foresee needing near 50 channels to have a truly explosive experience.  So far my plan involves using shift registers - which I'm quite comfortable with working on, but this board would simplify things greatly...That, or I pick up an arduino mega.

Then it's just a matter of writing sync software.