You have a an excellent Samsung 830 Series SATA 3 6Gb/s 128GB Solid State Drive. Samsung has a stellar track record. I do not recommend upgrading at this time unless you need more capacity. If you are on a tight budget have you considered a large capacity hard disk drive to store data files?
The ADATA ssd's are okay but they use an older SandForce SF-2281 Controller. SandForce hasn't come out with any new controllers in over 2 years. The current trend is to not purchase ssd's with SandForce controllers.
I maintain the ssd database listed in the sticky at the very top of this forum section. Here is the link:
Scroll down to the brands and models you are interested in and follow the links to the technical reviews.
I would disagree with your last coment. The 840 Pro and 840 Series ssd's are relaively new. They were released just 4 months ago. A few weeks ago newegg had a special promotion. The Samsung 840 PRO 256GB ssd was on sale for $199.99 and free shipping. The best place to check for special promotions and daily deals is over at Logic Buy. Here is a link to the ssd/hdd page:
Just the operating system? Any other software applications installed on the ssd?
If you are using one of the Microsoft Windows versions, then Windows should be using somewhere around 20GB to 22GB. Did you turn off hibernation, disable or reduce the size of page files, and change backup/restore settings so that backups are stored on a hard drive instead of the ssd?