What's the question?
First problem here is what is to be considered speed.
- Random access time?
- Cycle time?
- Maximum memory thruput?
- Average memory thruput?
- Either value per chip or
- for the whole memory subsystem?
For the following I'll go with maximum memory band for the whole memory subsystem. That is when a memory page is opened and successive access is done at full bus speed. In this page opening time can be (mostly) ignored, as it is (at each time) the same, independent of interface technology. Doing the numbers is thus rather simple:
RDRAM in the N64 is bytewide (*1) and operates at 250 MHz, delivering two bytes per cycle:
250 MHz x 2 Byte = 500 MB/s
EDO-RAM in the Playstation is 32 bit wide and operates ~33 MHz, delivering a word per cycle:
33 MHz x 4 Byte = 133 MB/s
So basically RDRAM as used in the N64 is about 15 times faster than EDO-RAM used by the PS. Considering the 4 times wider memory word of the PS leaves still leaves almost four times the memory bandwidth.
There are several benefits from using RDRAM with its high clock rate:
- Memory doesn't need to be as wide, as four consecutive fetches (for a word) can be done faster than the CPU can swallow it
- Smaller memory width means less lines to be routed. 9 vs. 32 bit results 23 saved lanes to be routed to CPU and graphics - at cost of only a single additional address line.
- Less lanes to route means cheaper board production - part of the reason why the N64 could get away with a simple two layer board
- Fast transfer speed allows a Unified Memory Architecture (UMA), saving the need for additional logic/buffers to separate main and video RAM.
- UMA allows flexible allotment of RAM to either purpose.
- UMA nullifies (or at least greatly reduces) the need to transfer data from main memory to video memory ans such can be done by bending pointers.
A unified memory architecture means that CPU and graphics hardware compete for RAM access. Depending on how much data either component needs to access, congestion can happen. Albeit, the quite high bandwidth and the way memory is accessed make it manageble.
Why EDO RAM in the PS?
At the time of the Playstation, EDO was already an outdated technology, passed by SDRAM were PC100 became standard in 1992. 32 bit wide SDRAM would have provided 400 MB/s. But as well be expensive.
Sony picked a design with separate main and video RAM so main RAM had only to go along with the CPU speed - and needed to be 32 bit wide to for sufficient thruput. So chips may have been way less expensive, but at the same time board design was more complex and it did need additional logic.
*1 - For simplicity we ignore the dance around the 9th bit some N64 fans love to do :))