1) Is that power supply aqequate for the rig I´m planning?


It will do the job, but you have not really thought about future expansion - you are probably better off with an 850w modular PSU

2) Is that graphics card worth the upgrade from the 1024MB 8800GT or is the difference really not there atm?


The 9000 series is not that better to warrant the price difference

3) I´m planning to put the OS on the 500GB hard drive and running two installations of WoW (one for the main, another for the slaves) on the Raptor drive. Is that the best idea for performance?


I would put the OS on the Raptor rather than the 500 if you are looking to upgrade to vista - as it accesses the hard disk far more than XP

4) Processor wise am I seeing a worthwhile difference from the 6600 that´s recommended for multiboxing. The rig will be used not just for WoW but for future games/music processing apps so is it worth spending that extra for the difference?


The 6600 G0 will see you quite happily for what you need to do regarding multi boxing - I wouldn't waste your money on the other chip

5) I´ve read something about problems with SLI (not somethingI know much about) with Nvidia cards - am I affected by that or is it only if you try to run 2 cards in XP?


you can run in wow in SLI - but if you intend to run multiple instances then SLI is NOT the way to go, as you will get better results running multiple monitors from your second card and a single monitor on your main card (for you main character) - SLI works by utilising both cards to cut down on the rendering on one monitor

6) I was determined not to upgrade to Vista for a long time given how long it took for XP to get its act together but I understand there´s less driver support for XP64 and I want that 8GB - I also do some heavy audio processing so I can´t get enough RAM. Is Vista chugging along pretty well now or should I go the XP64 route regardless? (My spidey-sense tells me I´m opening a can of worms here, heh heh)


I run Vista Ultimate 64 with 8GB of RAM ( I have a hot swap cradle with an XP installation that I hardly ever use any more) as the more ram you have helps. People complain that Vista is a worse OS, this is true in many respects (ie the security model, idiot mode for doing most things), but i have found that in most cases it is faster on my system than the Standard Xp build. My Q6600 runs 5x instances of WoW quite happily with no problems. people have mentioned that as WoW is a 32 bit program it cannot access more that 3.5 GB of RAM, this might be true, but as you are running 5 copies of it at once you will not be assigning that much ram to the application, so running a 64 bit OS will help you greatly, as you will have more memory available for other processes, that you might wish to run. people will also mention that problems with 64 bit drivers - the only problem I have found is running iTunes and trying to sync with an iPhone!! (hence the XP build on another disk )
As for the Audio side - I have spent the best part of 15 years working in the industry (from Atari's to present day) and having 8GB is a godsend when you are loading loads of VST's

I hope that some of my insights help you in your decision