64 bit CPUs can address more memory and usually address it faster. X86 64 bit CPUs have additional registries and specialized op codes.
None of this helps a router shuffle 1500 bytes through its small buffer from one interface to another. The mipsbe processor is well suited to network operations due to its RISC design.
The router only works with small amounts of data at a time so even tho it may process gigabits of data a second, the extra address space of 64 bit CPUs doesn't help since it doesn't look at all of that data all at once.
Putting 3 to 5 cards in the same case will kill your wireless performance due to RF noise, not lack of CPU power.