Ah, I had a brain freeze and was thinking "Gigabits per second"
My personal advice is 250GB SSD, plus a 2TB HDD (UPDATE: 525GB + 3TB). That's plenty for most people.
GAMES benefit mainly from faster load times. Sometimes it's significant, but usually not. Even if it takes an extra 20 seconds on an HDD that's usually just the initial LOAD of the game with occasional level loads taking less time.
SKYRIM and other games that load a lot (like buildings, dungeons) work best on SSD because even one second is significant if it's constant and helps with immersion.
You can always add a 2.5" SSD if you need more space. I'd also NOT PAY anything extra for a faster NVMe drive for gaming and other tasks. It's money thrown right into the garbage because you need a video edit workload that constantly stresses the SSD to separate SATA3/6Gbps SSD's from faster NVMe drives.
I can't make a case for 120GB SSD's because 250GB's are not that much higher in price.
2.5" Samsung 250GB ($90USD):
http://pcpartpicker.com/product/Ck98TW/samsung-internal-hard-drive-mz750250bw
2.5" SanDisk 480GB ($130):
http://pcpartpicker.com/product/x28H99/sandisk-ssd-plus-480gb-25-solid-state-drive-sdssda-480g-g26
Crucial M.2-2280 525GB ($150):http://pcpartpicker.com/product/34Nypg/crucial-mx300-525gb-m2-2280-solid-state-drive-ct525mx300ssd4
Crucial M.2-2280 MX300 ($90):
http://pcpartpicker.com/product/KXkwrH/crucial-mx300-275gb-m2-2280-solid-state-drive-ct275mx300ssd4
3TB HDD: $90:
http://pcpartpicker.com/product/j28H99/seagate-barracuda-3tb-35-7200rpm-internal-hard-drive-st3000dm008
Summary:
Prices are now LOW enough that the 525GB SSD plus 3TBHDD is only $240 plus tax/shipping. That's incredible!!
So I'd do that, and
a) install Steam to default, C-drive
b) add Steam folder to HDD (in Steam library settings. Make it something like "E:\Steam")
c) put most games on HDD, and games like SKYRIM that benefit most from load times on the C-drive (you can choose the folder when you start the DOWNLOAD process)
d) you can MOVE games between folders as well. (and I'd backup games to the HDD backup folder that you MOD so you can reinstall the game without redownloading)