The WEI tool in Vista is a general guide, rather than a serious benchmark. It measures total throughput for each subsystem, rather than the speed of any single component. Not a bad approach from a "Total System Performance" point of view, but as anyone can tell at a glance it's far from anything you could consider precise.
As I inferred: In the case of memory, the score you are seeing is a measure of how much data flows through that entire subsystem. Tighter timings would help some, sure. But from that 'total throughput' point of view your Front Side Buss speeds are going to have much more influence over this score than the clock/timings of your DIMMS. For Example, a 400 (1600 Mhz) FSB and DDR2 800 at 1:1 will max the score at 5.9. While running even 1066 RAM on a 266 (1066) FSB will get you a score in the high 4's~low 5's.
In the general sense, though, your setup is fine. Though the retentive types would try to tighten the timings some.
Well, IMO tighter timings (lower latency / higher spec) RAM will help increasing performance, esp. if you're doing some really memory intensive applications like image/video editing. But in case of normal gaming, I don't think you'll notice the difference.
Personally I don't care with those Vista rating My laptop got overall performance score 3.0 (due to it's NVIDIA Go6150 onboard graphic), and my friend's got score 4.3 (w/ Intel onboard X3xxx). But in actual performance, my laptop performs better (w/ better response feeling) than his.