Okay, I run 64 bit vista, and I have slightly mix-and match RAM - 4GB of it is Corsair 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400), and 1GB of it is A-Data, of the same compatibility. However, the Corsair runs at 1.9V and the A-Data runs at an unknown voltage, though I'm sure the 5-5-5 timing I have set is fine.
Anyway, upon changing the voltage from 1.8 to 1.9, Vista's Computer Rating Utility ranked my RAM from 4.6 to 4.5. I'm annoyed that my RAM isn't being ranked higher, seeing that every other aspect of my system is rated as 5.9. Is this Vista's fault, or my RAM's fault?
The memory score in Vista is based on the throughput of the entire system, not the speed of your DIMMS. On an Intel system, the bottleneck is the speed of the Front Side Buss - So as an example: DDR2 1066 running on a 1066 buss scores lower (5.5~ish) than DDR2 800 on a 1600 Mhz buss (5.9).
It's a nice guideline, and I like that it looks at the system as a whole, rather than measuring the speeds of individual componentry. Having said that, WEI is not by any means a serious benchmarking tool. It's merely a means to generate a general rating. So if you want a more reliable/exact score as to how your system compares, I would urge a proper tool.