CPU upgrade from i5-6500 to i5-6600.
Also a better CPU cooler.
Changed MoBo to support M.2 drive.
Double the RAM (16GB)*.
Switched 2.5" SSD to an ultra fast M.2 NVMe SSD (while gaining 8GB).
Replaced HDD for a more reliable one.
Picked cheaper case but as good as the old one.
Put in a great PSU that comes directly from the best PSU OEM.
Lastly, a 24" monitor to replace the 23" one.
And all that greatness under $1000.
* Note: Even though RAM speed is 2666 Mhz, it will be downgraded to 2133 Mhz, while working just fine.