What will your parents be doing with the computer? Will they be gaming or just internet, email, and applications (i.e.Word,Adobe, etc.)?
If they won't be gaming, I would suggest getting a new AMD Trinity APU or even an i3. The on-chip graphics would be more than enough.
Otherwise, if they will be doing some gaming (hard-core games, not games on Yahoo or Facebook) then your setup should be fine. Although I like Poltregeist's suggestions.
I second this decision, but if they are gaming, then I would recommend the HD7770 over the 550Ti.