Wouldn't even just one be insanely useful for decryption? I've been lost by pretty much every description of what they actually did at Bletchley Park (anyone have recommended links?) but it seems like they could do a lot with some decent computing power.
Poking through Wikipedia's entry on Colossus led me to http://www.bletchleypark.org.uk/news/docview.rhtm/487682 , which has this quote: "My laptop digested ciphertext at a speed of 1.2 million characters per second – 240 times faster than Colossus. If you scale the CPU frequency by that factor, you get an equivalent clock of 5.8 MHz for Colossus. That is a remarkable speed for a computer built in 1944."
It should be pointed out that a linear scaling is inappropriate, though, because Colossus was not Turing complete, which makes direct comparisons very tricky. The very oldest things we today call "computers" tended to do a lot of stuff in very, very specialized hardware, and converting that into a modern-day, very general-purpose "clock cycle" is not trivial. It might be better to understand this as a glorified GPU-like processor. Only even less powerful, as it was even more special-purpose than a modern GPU.
(Or at least it will be better, if you understand the intrinsic limitations of GPU-like processing. If you're a Cell fanboy who bought the Sony's PS3 gibberish about performance hook line and sinker and still haven't worked out why Cell-based computers haven't destroyed Intel without invoking some sort of paranoid theory, that might not be as helpful a way of understanding things as I'd like.)
