I fear you are dealing with poor quality power supplies.
Dynex is a tier 5 unit(not recommended, replace asap) on this list.http://www.eggxpert.com/forums/thread/323050.aspx
The thermaltake may be no better, depending on the model and sample.
If a psu can not deliver the necessary power, the graphics card will not run full out
Your GTX550ti needs only a 400w psu.
A gtx660ti needs only a bit more at 450W.
Here is a handy chart:http://www.realhardtechx.com/index_archivos/Page362.htm
Since the dynex was working OK, you might as well keep it.
In the future, look for a tier 1 or 2 card.
A psu is not a good place to economize.