Your system will be fine for 5-boxing.

The single largest upgrade will be an SSD drive.
One of the cheaper drives, purely for the gaming folder would be ideal.

After that, the ram would be the next upgrade.
Or, if you're doing a lot of multitasking or video recording of your play, possibly the ram first.

I'm using a GTX 275 video card, and wow does not push this card.
I also play with view distance at half, and shadows disabled.

My processor is an i7 920, at stock speed.
And again, warcraft does not push it at all.