One thing to remember is that the Celey Tu will be bandwidth limited since it cannot take advantage of DDR capabilities. When set at the same clock speeds as Athlon XPs or Pentium 4s, the Celey Tu compares favorably in benchmarks, even beats in some. But anything dependant on memory bandwidth will reveal a substantial lag for the Celey Tu.
Still, I was able to pick up an Abit ST6 mobo and Celey Tu 1.0A for a total shipped price of $131. So I am doubling my clock speed for that price. That is cheap. If I wanted to get the bandwidth improvement, I would have had to buy memory and a new case/PS too, which would have more than doubled my cost. So for folks wanting a cheap upgrade to a "decent" speed for cheap, that seems to be it. Otherwise you will have to spend twice that. My next upgrade will be to a better vid card, either a GF3Ti200 (cheapest) or GF44200.
I'm following Oldfarts advice and OCing the 1.0A to 1.333 or 1.4 ghz... that should yield better performance than a 1.2 set at a lower FSB or even a 1.1A... the reasoning is you want to get that FSB up has high as possible to help the memory bandwidth issue. You still top out at the same approximate CPU speed with all these chips, so the FSB/memory is the thing that helps make the difference.