Stock trading I don't think requires lots of power in a rig. My father does some stock trading on the side (just with a single monitor setup) but if you are one of those hard core stock traders. You probably want an SSD for fast boots and a dedicated card for Eyefinity.
So I suggest going with the build in my siggy for the 300$ computer. Instead though, swap out the CPU for a Phenom II 925 and get the 5450 I mention that way you can Eyefinity 2 monitors.