For multiboxing, more cores is usually always a better bet than more speed. Games are mostly single-threaded, so a single game will do better with a faster and lower-core-count CPU. However, we're playing multiple games, so multiple cores is usually better. Your OS and background tasks will eat up some CPU usage, so consider that 1 core. Each instance of WoW can utilize a core, so you're better off with the Q6600.

With that motherboard choice, you don't have a choice on RAM - it uses DDR2, which is not compatible with DDR3 (different physical slot even though the same pin count - it won't fit in the slot). Unless you really like the Wifi option, I'd personally go for the Asus P5Q-E P45 board (it supports up to 16GB DDR2 memory, instead of just 8GB DDR2).

Keep your old 74GB Raptor - it'll still have lower access times than a 7200rpm drive. I was using my 36GB Raptors in RAID0 for a long time while 5-boxing. You don't need multiple hard drives for WoW, just use multiple WoW directories with sym-links. Read the Wiki on that topic for the performance benefit of links instead of separate files.

The videocard is really up to you. A 24" monitor is a lot of pixels to push at 1920x1200, so a faster card with more video memory will help you more than someone with a 20". All the new cards are very fast for an old game like WoW, but for the future.. who knows? Faster is always better for future-proofing, unless you're open to the idea of upgrading the card later if you need to. The 4850 should do you fine for now.