Stephen,
Make your choice this way: Or think about this way.
You will either want CAS 2 or CAS 3 ram in SDRAM.
Corsair CAS 3 ram will get you about or around to a 146 fsb, depending upon several factors. Crucial or Mushkin CAS 2 will get you within 5 mhz fsb. Forget anything else, too unrealiable or unpredictable.
CAS 2 Corsair will go to about 175 mhz fsb, maybe higher, depending upon system. I have read of peeps taking the PC 150 Corsair all the way into the 190's! If that is true.
CAS 2 ram from Mushkin will do similar. Crucial will come within 5 mhz fsb, again. Forget anything else.
If you go DDR you can use Cosair CAS 2.5 ram just as well if not better than you can any one elses CAS 2, including Mushkins or Crucials. Meaning no need to spend extra money for Corsiar or any DDR CAS 2 ram.
All three makes; CAS 2 for Crucial and Mushkin, and CAS 2.5 for Corsair will all top out in the 160's. CAS 2 Corsair is in a class unto itself. Not that much faster than 2.5 but if one wants the extra speed or leeway, it is there. Again I wouldn't consider anything else but these three makes of ram. BTW I've read that the Corsair 2.5 will go/give 333 DDR speeds right now, for the new 333 DDR ram that is about to be introduced into motherboards.
Now consider what you are really most likely to get from your system, concerning fsb speed. Most average oc'ers will be looking at the mid 150 to the 160 rage for their fsb speeds. Some less, some more, depending on many things.
What you want to consider is the amount of safety net or margin for future changes, as you suggested above. We just can't know what the future will bring.
My brother for instance has all PC133 CAS 3 ram. He bought his last stick less than 3 months ago. Now he desperately needs to oc his system further than his ram can go. Yet he is stuck with a under purchase, not considering his computing future. I tried to tell him at the time, but?
Also consider how long you intend on keeping your ram before upgrading again. If you intend to keep it over a year, especially 2 years, get the best you can, or the best clocking ram. Because a lot can change in a year, and also that ram may be worth just a little if it is real good to begin with, cause reall good ram is worth a slight premium.
You should be fine with the Crucial, but if you want it all, and know you can get no better, then go with the Corsair. the Crucial CAS 2 SDRAm or CAS 2 DDR should oc all you need, but with a slight limit compared to the Corsair if you go real heavy, to the limits oc'ing your ram.
Some of the website reviews may slightly vary from my statements, as new ram tests slightly differ all the time. But I have given you a good idea of the generalities of what to expect. Unless something drastic changes.
Cheers,
Mike Lamb