Back to the OPs questions:

1. For $150 the best upgrade you can make to your current system is more RAM. I run a 12GB system very similar to yours and sit at 8-9GB utilisation.

2. You can perform this upgrade by either purchasing a 3 x 2GB kit (pick something that is rated to 1600MHz ideally) or a
3 x 4GB kit. The 2GB kit should slot into your empty slots quite happily as long as it is decently rated (ie. not 1066MHx or anything stupid). As I think Fenril mentioned mixing memory can sometimes have pitfalls. With your existing motherboard in Tri channel setup, adding a new 3 x ? kit should not be a problem. If you were to mix different DIMMs in the same channel you will run into problems but normally this may just reduce you to dual or single channel efficiency.

Personally I wouldn't worry about the 3x4GB kit if you want to go for 18GB total. Currently WoW is not allowed to use more than 2GB per instance so you will never use more than 10GB unless you have other programs open which also use a decent amount.

Where the 3 x 4GB kit would be nice though is if you want to overclock your CPU. I run my 920 at 4GHz very happily on air 24/7. The gear that you have should support this quite nicely. If you had all 6 of your DIMM slots full however, the memory subsystem on the X58 motherboards can often be a weak point for overclocking.

In summary if 3 x 4GB ram fits into your budget, then pull the trigger, no questions asked. Once you have the 12GB you can then play around with OCing the CPU at your leisure (even on the stock intel cooler).

If you would rather save the cash, go for 3 x 2GB for 12GB total.

I agree with the other guys, once you have the ram, save for a video upgrade next, but anything less than a 5xxGTX is probably not worth the cash. I also like NVIDIA over ATI for WoW performance in general. Check out the Tomshardware WoW performance summary if you want to compare hardware for single wow instances.