Good graphics card upgrade option includes the GTX 960 or the pricey GTX 980Ti (at $680 its a bit steep). Unlike other HP desktops, the Envy Phoenix already comes with an upgraded power supply so you won't need to get more power when you select the better GPUs. Lastly, the Core i7-6700K at 4.0Ghz for $40 more is a good upgrade choice too.
Add the following components to make this build and get this price:
During checkout cart, look for the "Add" coupon button on the bottom left to insert the 30% off HP Envy 750se coupon above.