I would not read too much into the Intel onchip/onboard graphics. It is just not that important in trading applications. The graphics performance may be more important for laptop users, who relies more on the onchip/onboard GPU. On a desktop, you can easily buy an external video card (some are very inexpensive too), and probably would disable the onboard graphics any way. In trading, drawing candles and squiggy line indicators are all 2D applications. Not graphics processor challenging.
The Intel website confirms an external video card would disable their on board graphics. It seems even the lower end multiple display cards would be at least as good if probably not better than the on board...so the only other question is whether you`d want even better to deal with eye strain issues. I`ve been able to find threads on monitors here and higher resolution LEDs with high contrast and low response time appear to be the way to go. But I have no idea which cards would suffice.
The Intel HD Graphics are aimed more at laptops then desktops. Using the built-in graphics consumes less power then an additional add-on graphics processing unit. Sometimes the HD Graphics does work together with the onboard graphics processor. In regards to desktops, a decent graphics card is anywhere from 75-125 dollars from Ebay. Any decent graphics card will run rings around the onboard graphics. Since you are not worried about battery power on a desktop then the onboard graphics is not really a concern.
Here is something to consider. The more powerful the computer the greater the need for noisy fans and such. Some video cards have an additional fan. Guess what? You are going to be sitting by that thing everyday listening to the fans. Sometimes getting the best performing machine possible isnt the best solution. I would go down to the store and listen to what you are getting yourself into first. Another alternative is to locate the desktop in another room or the closet if you can to cut the noise.
Most traders would be surprised to learn how little the CPU and GPU work in a trading environment. Usually better to go for a quiet, adequate machine than a high-power (noisy fan) rig.