The risetime give you the ultimate bandwidth of the signal, its repetition rate give you the sideband distribution within that spectrum. So a 1KHZ square wave with a nS rise time would give you a comb of frequencies at 1KHZ difference right into the GHZ, but each individual harmonic would be a a very low amplitude(1/n). A 20 MHz square wave with a 8nS rise time would give you multiples of 20 MHZ.
So its the clock (or R/W pulses) multiples that will cause you grief up to the b/w limitation set by the rise time.
Frank