Nice move up to the 64-bit OS! In my experience, the bump up to 8GB of RAM made a big difference in terms of performance when running 5x clients at once.

However, be sure to watch out for potential display/GPU problems from the nvlddmkm.sys error (or its ATI equivalent depending n which card brand you use) which is endemic to 64-bit version of Vista. I was running Vista Ultimate 64x SP1 for about 3 weeks and sudden began receiving a 'the driver has stopped responding but has recovered' error whenever I fired up even 1 window of WoW, much less 5. It has made WoW basically unplayable on my desktop machine.

I'm still trying to find a satisfactory driver fix for it, but this problem is fairly widespread amongst 64-bit Vista users it seems from what I've read on the Nvidia forums (113 page locked complain thread; the another rapidly filling "new" thread about it). If you run into any problems with this type of error, private message me and I can point you in the direction of workarounds, etc that I have found over the past few weeks.