I doubt WoW uses the CPU for much at all, just some calculations, why do you think the CPU is pegged even with 8 clients? Lax would know better then I would though, but I think a lot of posts show people getting good results going from 4-8G ram rather then moving to I7 cpu.
There is a pretty good amount of CPU work going on, I'd say it's easier to be CPU bound than GPU bound for example (given the graphics settings can be put in performance mode). All of the Lua built into the game interface, including Addons, will be running on the CPU, and each frame the game needs to do a nice handful of calculations to move characters, check distances, handle queues of I/O, etc. The thing about RAM is that you need to run out of it before adding more (all else equal -- more bandwidth/faster clock is a different issue) is going to be largely beneficial, but you can always find something to do with a faster CPU or more CPU cores. All that said, I'm not sure why going from 4 to 8 GB would be much of an improvement compared to doubling CPU cores, unless they were running out of available RAM. I'm with you on the putting WoW data folder in RAM for anyone who wants to get what today is a massive amount of RAM though If I had the money to spend I'd do some benchmarks on that.

I'm not sure how performance compares between the i7 and a non-hyperthreaded quad core, but I wouldn't expect it to perform as well as having a full 8 cores, all else equal. Benchmarks on every site I see/know of seem to be single instance of various games and benchmarking standards. Maybe we need to figure out how to set up and fund a multiboxer benchmarking studio so all these questions about how good systems are for multiboxing can be properly answered with some hard numbers and as few variables as possible between comparisons instead of random person A's PC+options versus random person B's PC+options

On topic: The system with the given specs will probably perform pretty well. I'd recommend trying Windows 7 64-bit (free until next year!) and compare your results to your other Windows flavor of choice.