Thanks to MS for trashing hardware audio support through DirectX. If you want an aftermarket sound card, I'd go with a Creative Labs X-Fi card and download their Audacity(I think it's called?) package. It's basically a bunch of software wrappers for using Vista's OpenAL support to directly access external sound hardware from DirectX games - WoW is supported. Without that, even with an aftermarket sound card, it won't be used - Vista will still use the CPU for software sound processing. Note that currently only the X-Fi series is supported by Audacity.

Based on those specs though, it's most likely your CPU slowing you down with that many instances of WoW. The Core 2 Duo architecture is that much better than the X2. I had my E6600 at stock speed running 5 instances in XP with 4GB memory without fps issues on my main screen (4 alts are now capped at 15 or 20 fps). I overclocked it to 3.2GHz and it bumped the fps even more. I do have an X-Fi Elite sound card though, so that does help with the audio processing being off-loaded from the CPU..