you don't want to have your movies on ssd, it's pointless unless you have some sick 1gbps bitrate videos which is a bit of a stretch since it'd imply you're dealing with 4k uncompressed videos
120/128gb ssd is enough for os, all your apps and few games you play frequently (remember to leave ~15% of space free so garbage collection can do it's thing, else you'll have severe performance loss)
keep your movies and all the multimedia stuff and documents on hdd
about improvements you can expect to gain, blazing fast boot times, instantaneous app launching (or severely reduced loading times of bigger production apps such as photoshop, sony vegas etc.) and overall snappy system responsiveness
when it comes to games, you can expect faster loading times and games such as wow won't have trouble with drawn out loading of characters in crowded places