Some improvement, yes.
Bench tests will not tell you how well you will do. It is entirely dependent on YOUR access patterns, and how much of your hard drive data can be held in the cache.
You will benefit only from reads to data that happens to be frequently used and found in the SSD cache.
Writes will have to be passed through to the hard drive.
It would be better, in my opinion, to use the same funds and buy a SSD on which you install the os and some apps.
The same price buys you an Intel 60gb 520 SSD or a Samsung 830 64gb ssd.
the problem is i need at least 150gb. And with this ssd i can perform a full hdd encryption without problem.
Mostly i use it to develop application, play 1080p movies and play battlefield3
I am no expert on drive encryption.
The Intel SSD has presumably good capabilities in that area.
A 60gb ssd will hold the OS and a number of apps.
It will make everything you do feel much quicker.
Use your current 500gb hard drive for storage of movies and such.
If all you need is 150gb, then consider a 180-240gb drive and put everything on it. It will cost you about $250-$300.
You will have no regrets about the speed.
Put the 500gb drive into an external enclosure and use it for backup or archiving. You do have external backup for your critical data don't you?
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
If you need full drive encryption for work, I would dual boot with my work drive and my personal drive. The data migration software that comes with Crucial's drives works great. It'll take you under an hour to make a clone of your current drive, even when going to a smaller drive. Also assuming you won't have issues with the current encryption.