A couple things:
The first is that most motherboards are set up for RAM to be paired in every other slot - i.e. slots 0/2 and 1/3, rather than 0/1 and 2/3. So you may be running in single channel mode. That'd be worth around 5% performance right there. Check to be sure your mobo is supposed to be adjacent slots and not every other one. Make the appropriate change, if needed.
The second thing is WEI measures the performance of the entire subsystem, rather than the speed of individual components. I tend to agree with the view of looking at the system as a whole - It makes little sense to me to install fast components and end up running them below their potential due to not paying attention to setup details. Having said that, WEI is *not* meant to be a comprehensive benchmark, rather it is just a guideline. There are other much more effective and detailed benchmarks if you need that.
Details on WEI, if you care to read it:
http://windowsvistablog.com/blogs/windowsvista/pages/458117.aspx
Since WEI is measuring the throughput of the entire subsystem, in the case of Memory on an Intel based system the front side buss is the limitation and not the speed of your DIMMS. In other words: Your PC2 8500 (DDR2 1066) stuff is being held back by a 1066 buss. I have my own memory running at 800 Mhz and score a 5.9 - But I have the FSB cranked up to 1600, rather than the stock 1066. Tightening the timings would help a little, but bumping up the FSB would make a big difference. Of course, doing so would also overclock your Processor.