Honestly that system probably still outclasses all but the high end macbook and mac pro. Get a 512 GB SSD. Don't look back. The SSD boot drive + SSD cache makes a huge difference in performance and render speed. SSD's last longer than HDDs anyway. I'd highly recommend the Samsung EVO pro line, I get over 600 / 500 MB ps read / write on a single SATA III drive.
I recently did that same upgrade to my oldest workstation that now is my home theater pc / server, and it went from over a 3 minute cold startup to under 40 seconds