"Reliability and Performance Monitor." You need to use it. It will tell you where your system is slow.

I 5box one 1 machine running vista x64 (core i7 920 with 640gb HD, 6gb RAM, asus 4850 w/ 512mb ram). It ran fine (60fps) with 6gb of RAM, but I was dropping some frames in shat, so I ran the reliability and performance monitor for an hour. The report it generated told me that i had a lot of memory hard faults (pc requests data from memory that has been paged to disk), and recommended either adding more memory, or reducing the demand on system memory. I decided to add another 6GB (12GB total now) to fix it, but it wasn't necessary (i run vm's & other). Obviously not everyone is going to want to throw money at the problem, but the built in pefrmon tools in vista will tell you where the system slowdown is, and what you can do to fix it.

For vista only:
1. open reliability and performance monitor
2. expand "Data Collector Sets" -> "User defined"
3. right-click "user defined" folder and choose "new" -> "Data collector set"
4. call it "wow" and choose "create from a template"
5. choose "system peformance" from the list and choose "finish"
6. right click on the new user defined data collecter set you created called "wow" and choose properties
7. under the "stop condition" tab, set an overall duration of 30 minutes and click "OK"
8. right click on the "wow" collector set again and choose start

In wow, go about your business as you normally would (dont just stay in one place, go in & out of cities etc, instances etc.). When the 30 minutes is up, it will compile a report that will be visible under "reports" at the bottom in the left pane. It will tell you where the bottlenecks are and give you recommendations on how to fix them.

I've run the last 3 months with 5 separate complete copies of the wow directory (d:\games\wow1, d:\games\wow2 etc.) and only recently created directory junctions (don't simply create a symbolic link, you should create a directory junction) on wow2-wow5 back to the data folder in d:\games\wow1. The report was showing me a high number of disk reads and a moderate disk queue length on the common.mpq and expansion.mpq files in each directory.

I created renamed the Data folder in d:\games\WOW2 - d:\games\WOW5 and created a junction with "mklink /J d:\games\WOW2\Data d:\games\WOW1\data". I haven't run with this configuration for long enough to report if there is any benefits, but it doesn't appear to make a huge difference initially - the game still has to read from the file 5 times, but vista my handle cacheing more intelligently with all reads pointing to only 1 file.

Like i said, i 5boxed on 6GB of ram on a 24" lcd @ 1920x1200 with max detail and it would only drop frames in the major cities. It was smooth everywhere else (granted, my i7 920 is a very nice processor). Unless your processor is slow, you should not be having to turn off all kinds of features in vista to get acceptable performance. Run the performance monitor and it will tell you what`s up.

Hope that helps.

*EDIT*
forgot to mention that my 640GB HD has 32MB cache, which was a major jump in disk performance from my old drive with only 8mb of cache.
I also set processor affinity for all my instances of wow with hotkeynet.