The probe and Oscilloscope selections for measurement
When choosing the right scope and probe for high-speed measurement, first consider: signal amplitude, source impedance, rise time, and bandwidth.

If rise time is an important measurement, then both low probe capacitance or low (near matched) impedance methods will work.  Then for rise time,tr the  DSO BW needs to be greater or faster the your desired measurement  BW=0.35/tr  . This is for 10% to 90% of the rise time not 64%.  

When using RC with R being driver RdsOn in Ohms and probe capacitance, RC=T can be converted to 10~90% rise time using tr=2.2T  then converted to bandwidth limitation using probe capacitance.  It is useful to know high speed low voltage "74ALCxx" CMOS is ~ 25 Ohms nominal and 5V logic is 50 ohms nominal.

If accurate rise time measurement is critical, then active probes are the more important than the DSO specs.  http://info.tek.com/www-abcs-of-probes-primer.html

But to measure all the parameters of DDR memory,  I would give serious consideration to a low cost used 8GHz Logic Analyzer with &lt; 1pF probes such as TLA5202 with 68 channels and 125 ps resolution with variable threshold.  ( market value $1500 used)