Upgrading the inspiron 531

the ultra psu looks suspect. i think it is a low end old psu. the voltage switch in the image means that it has no active pfc, could be old. imo never trust cheap, low end psus. power supplies are vital components for pcs.
you could buy a better, 80+ rated psu instead of the ultra one. this one should be able to support your gtx 550ti. check out this one:
corsair cx500 v2
http://www.compusa.com/applications/SearchTools/item-details.asp?EdpNo=422168&CatId=5431