THG did a review of the differences between 2.5 and 2
http://www.tomshardware.com/mainboard/20020507/index.html
In the bench marks CL 2 out-performs CL2.5.
If the crucial is rated for 2.5 I'm not sure if it will run stable at 2. (I've never tried it with that speed) And if it did, you may or may not "see" the differnce.
I'm not exactly clear on benchmark scores vs. what your eye can detect. The CA2 was <1 FPS faster in Q3 and in 3Dmark 2001 it was 13 points higher. I doubt I could tell the differnce between the 2, but I still want the fastest
Personally I run Corsair Twin512-2700LL (2-2-2-5) and I am very happy with it.
"Quotes are for people that can't think for themselves"