I'd be real happy with 640 MHz AVR's, buuuut they'd have to make fast flash as well as transistors at a minimum to pull it off.
Flash cells are a type of transistor. Hopefully the grapheme technology allows for a floating gate.
In flash memory, each memory cell resembles a standard MOSFET, except the transistor has two gates instead of one. On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (FG) insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years. When the FG holds a charge, it screens (partially cancels) the electric field from the CG, which modifies the threshold voltage (VT) of the cell (more voltage has to be applied to the CG to make the channel conduct). For read-out, a voltage intermediate between the possible threshold voltages is applied to the CG, and the MOSFET channel's conductivity tested (if it's conducting or insulating), which is influenced by the FG. The current flow through the MOSFET channel is sensed and forms a binary code, reproducing the stored data. In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG.http://en.wikipedia.org/wiki/Flash_memory
Anyway, the type of memory technology that the AVR uses doesn't really matter at all as long as it is reliable and allows one-cycle access. If it was implemented in some sort of battery backed up SRAM, as an E/E/PROM (as many microcontrollers memories were in the past), FRAM (new technology), or some new technology that works with graphene well, it wouldn't matter at all to either the application programmer or even to the CPU core itself.