According to your mobo you can have 8GB of Ram. You also have 2 PCI express 16 slots. Here is what I would do ->

Upgrade ram to 8GB
Upgrade OS to 64 bit

Graphic cards are a touchy subject and strongly depends on how you use them so I won't mention any upgrades for that.

The next thing I woud upgrade is the storage hardware. I would add an SSD just for WoW. All you need is a 30GB SSD nothing more.

After that I would look to see how the CPU is handling all this. If you are running above 90% with 5 wow clients then its time for a CPU upgrade.

Finally, the GPU upgrade is the last in the chain and is highly dependent on your setup and what software you use to manage them. For example I have 2 cards, each driving a monitor. I use InnerSpace to manage the GPUs so that both have about the same load on them. You may be running 1 monitor or even 2 monitors from 1 card.

More info on how the monitor/GPU setup can be helpful in determining an upgrade for you.

Drommon