First off - YES a 120/128 Gig is OK. My systems (2 desktops and 2 laptops) all have an SSD for OS +Programs and on average uses about 40 gigs. Have 10 SSDs no problems todate. The Samsung 830 or Curcial M4 would be an excellent choice. I normally highly recommend SSDs.
However, in your case I have some reservations.
1) Most users on this forum are gamers or into video editing. Very few are "causual users" who simply use the computer for Internet/email/watching video.
2) With the exception of laptops, the vast majority of Desktops here are setup with the SSD as a OS + program drive and a HDD for storage. And often High write volume is directed to the HDD.
3) MLC SSDs do have a write cycle limitation that in most cases will result in Long life. SSDs employ a "wear leaveling" algorthum that attempts to equallize the write cycles per cell. With a single SSD and if the drive ends up close to the 90% fill then the ability to equallize these writes is impaired, which could cause some cells to "die" much earilier than others - this then creates a condition that is cumulative in nature (like a degenerative disease).
4) My Son designed and sells a "box" that goes into a TV that allows for internet, email, streaming video (a computer). Units with SSDs have a much higher failure rate than units using a conventional HDD.
My Bottom line: In this case the SSD will ONLY improve boot time and loading a program and NOT improve watching the video, reading email, and surfing the web. If single drive, recommend a "good" HDD (Not one of the "green or Blue models). If can afford, a small say 80 -128 Gig SSD for OS + Programs coupled with a small HDD for downloads (the streaming video).