The issue is that power supply. I do not believe it's enough. Not only is it under the recommendation, pre-built computers typically included bare minimum power supplies, as cheap as possible. So you probably aren't even getting 240 usable watts out of it. You can try it of course, it might work, but I don't recommend it.
SFF computers are not good choices for videocard upgrades. The small size is a sign of many compromises in design and upgradeability.