Kartman,
My experience (admittedly limited compared to most who hang out here) is that memory rated for 2.0 above PC2100 (DDR266) is both rare and pricey compared to 2.5. I have also seen some SiSoft Sandra scores that clearly show that while the CAS speed may be a prime differentiator at 266 speeds, it has increasingly little impact as your raw bandwidth increases.
Specifically, a very small increase in bandwidth will more than compensate for the difference between 2.5 and 2.0, so given that you are going to overclock anyway, you are very likely to eventually run your ram at 2.0 in your desire to push it to higher fsb speeds. The CAS, like almost everything else, is a rating, rather than a protocol, and that rating applies to the native speed of the ram. So if you buy ram that is rated for 2.0 at 333MHz, and want to run your memory at 400MHz, all bets are off anyway.
For the record, my memory is rated for 2.5 at 333MHz, but runs just fine at 2.0 at 333MHz. But of course, I want to run my memory at 400MHz, and no way is it going to do 2.0 at that speed, rating or no, so it really doesn't matter at all in my setup that it can do 2.0 at 333.
Hope this helps. As always, if any of you hot shots see something in error here, pound it out of me publicly and mercilessly!
Nilonym