The difference between PC-133, PC-100, CAS2, CAS3 , wil there be a performance increase? - PC-133 memory is designed to run stabile on a 133MHz FSB. PC-100 is not designed to the PC-133 spec so it is very not recommended if you will get it to work 100% stabile, if at all, on a 133MHz memory bus. There is very little reason not to go with PC-133 as it is almost the same price as PC-100, and the nice performance increase from the increased memory bandwidth that 133MHz provides.

The difference between being able to run your memory at CAS 2 and CAS 3 is how much memory latency your CPU will see. You will notice a performance difference with a 133MHz CAS 2 module. This is because CPU's are now running many times faster than the speed of of the memory bus, so memory bandwidth is becoming a performance bottleneck for many applications and it's only gets worse with new and faster CPU 's that are released. As the CPU's speed increases it is wasting an increasing percentage of it's time waiting for data to be fetched from main memory. This can be quantitized by multiplying the latency of the memory system with the multiplier of the CPU. So a 500 MHz (5 x 100 MHz) CPU will see 5 (multiplier clock of a typical 500MHz CPU) x 9 cycles latency (typical SDRAM latency), meaning this CPU will have to wait at least 45 clock cycles for data that can't be found in it's L1 or L2 cache. If the SDRAM latency doesn't decrease (via lower CAS, RAS-to-CAS delay, etc...), then each faster CPU release keeps becoming less and less effective at increasing performance because it wastes more and more clock cycles sitting around waiting for required data. To put it simply, as modern CPUs run on increasingly large multipliers the impact of the memory subsystem is rising. To get the most efficiency out of your CPU, get PC-133 CAS 2 memory modules.

What is the difference between the Mushkin High Performance REV 1.5 & REV 2? - The latest revision of the module (REV 2) has more fine tuned traces on the PCB than the rev 1.5. This allows for a shorter path and less electrical interference (i.e. crosstalk). Less distance + Less electrical interference = clearer signal = better stability at higher speeds. Rev 1.5 was already stable at 133MHz with a CAS of 2. So the improvements in REV 2 will allow for more potential overclocking. So if you’re looking to clock higher, we suggest the REV 2 every time. If you’re looking to stick with 133MHz at CAS 2 then you won't see any performance difference between the two as it is running at exactly the same speed. (both revisions are rated cas 2:2:2 at 133Mhz)
As modern CPUs run on multipliers of 8 and more, and have faster but smaller L2-caches than predecessors (New Athlon versus Athlon, Coppermine versus Katmai), the impact of the memory subsystem will rise. So being able to overclock your memory will also be more important