PC150 sdram is factory certified to run stable at 150Mhz.
If you buy PC133 its certified for only 133.
but if you buy crucial or micron it may well reach 150. or it might not. all depends on the quality of the stick, who made it, what batch its from, and just how lucky you are.
personally i pushed my PC150 all the way to 160Mhz @ CAS2. the ram worked fine... shame my hard drive/PCI devices couldnt handle the elevated PCI bus speed of 40Mhz
as for SRD vs DDR:
DDR will give you a varying performance boost, depending on which type of DDR you use, which mobo, and what kind of applications you run.
AVOID the PC1600 DDR. go for either the PC2100 or PC2400 (PC150 DDR)
if you live in the states DDR costs the same as SDR, so you may as well go for DDR.
if you live in a technological backwater country like me, DDR still costs almost twice SDR, so you might wish to wait until a better DDR capable motherboard is available (the new SiS board or Nforce).
P.S. according to the sandra mem benchmark, my PC150 SDR @ CAS2 is approximately equivalent to PC2100 CAS 2.5 DDR (generic)
"i love the smell of Overclocking in the morning!" Says my Hamster.