I have similar gear and I can play 5 wows no problem:

CPU = Q9650
Mobo = Asus Striker Extreme
Ram = 8 GB Kingston @ 800 Mhz
Video = Nvidia EN8800 GTS (strangerly performs much better than the 9600)
HD = Main system HD + one extra HD + 5 other HDs in RAID 5. All are Seagate 500 GB HDs. I use the main systen HD for the main wow and the 4 slaves are on the extra hd that is not raided. The data folder is symlinked on the slaves HD.
OS = Vista ultimate 64 bits
Sound = SB X-Fi. I don't use the sound card that comes with the mobo because it draws some performance and the drivers that come with it suck big time. Also the sound queality it delivered was pure crap. It was called HD audio or something like.

The case I use is a datacenter designed one with lots of ventilation and airflow. As long as the machine runs cool the performance is good.

Make sure you use the correct dirvers and don't use the ones that come with the mobo CDs. Go directly to Nvidia and download the needed drivers from there. Install only the drivers you need and not the stupid asus republic of gamers drivers, these seem to cause strange behavior when running in crowded areas.

A big performance boost for me was adding more ram and separating the main wow from the slaves on a different HD. It takes a bit longer (about 2 seconds) to load dalaran, but during lag spikes the main performs ok and the slaves seem to fire stuff reagrdless they are lagging on fps or not.

You can handle 5 clients with 4 GB of ram, but you will be unable to approach cities nor raid. Basically until northrend you will see not problems. Once you reach wolk content you will need to add more ram because the zones are too big and require more memory to load properly.

Perhaps you will need to add another video card for the second monitor. I run mines on a single big monitor... moostly because I have already other two 24" monitors on my desk and addig a fourth one would be not that cool and would need to get my head spinning like the girl in the exorcist movie.

I have all my slaves forced to 15 fps when inactive and 30 fps when active and the main completely unrestricted. My main rarely drops under 30 fps. The only time I have seen my main under 30 fps is when dalaran is very crowded during peak hours and in wintergrasp zerg when there are lots of people (two raids on each side).