Here is a good starting point on
upgrading, Samsung 840 Pro seems to be the leading SSD currently. 250GB SSD also seem to be the sweet spot regardless of what brand you get.
And Yes an SSD if the best upgrade you can do it is very significant, If you haven't worked with one you'll find it amazing.
Whether you upgrade to Win 7 or stay with Win 8 is personal preference, many users are very happy with Win 8, many of us are happy to stay with or upgrade to Win 7
If your going to stay with Win 8 you most likely will benefit from
Classic Shell or if you rather a simpler paid for start button Start8. Either program will get you back a start button. And by pass the start screen on boot up bringing you straight to your normal desktop.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro