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).
Here's the lowdown on what WEI is and what it's measuring:http://windowsvistablog.com/blogs/windowsvista/pages/458117.aspx
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.