i personally don't raid anything, but that's me.
i have a 128gb drive, which is the only drive in my machine. after the OS install i had a little over 100gb available, which i install everything on. if you plan on putting lots of games (or very large games) onto the drive, i would consider going larger than 128, as it gets used up pretty quick. i personally wouldn't put any photo storage or large documentation on it either...grab an external drive, whether an enclosure or just a thumb drive, for those types of requirements. i use a router-attached USB enclosure to hold my network storage.
SSD makes a significant different for load times on games - probably 10-25% of the load times from normal 7200rpm drives(from my experience).
Lian Li Lancool First Knight Series PC-K59W
Intel Core i7-960 Bloomfield 3.2GHz
EVGA X58 FTW3 132-GT-E768-TR
EVGA GTX 660ti
G.SKILL Ripjaws Series 12GB (3 x 4GB)
Crucial M4 128gb SATAIII SSD x2
Crucial M4 512gb SATAIII SSD
Corsair Professional Series HX850 PSU
Dell UltraSharp U2713HM 27" w/drop ceiling mount
Sennheiser PC 350 Headset
Saitek Eclipse II