PSU calculators are accurate, but we don't know and can't exactly what to put in.
A good old fashioned ROT is plenty good enough.
The psu requirement is almost entirely gated by the graphics card/s you will use.
In the case of a GTX550ti, you will need a 400w psu with 24 12v amps and a 6 pin pcie power connector:http://www.evga.com/products/moreInfo.asp?pn=01G-P3-1556-KR&family=GeForce
500 Series Family&sw=
No harm in getting a stronger one up to perhaps 650w.
Quality counts. My short list of quality psu's would include Antec, Seasonic, XFX, Corsai, and PC P&C.