Your computer will only pull the amount of wattage it needs, so if it only needs say 300w you can use a 400w psu just as you could use a 600w psu.
Now if you get TOO powerfull of PSU then you start running into efficiency concerns where a 800w psu is so bellow its efficiency range that it actually takes 400w for that 300w load while a 500w psu would only use 345w for the 300w load.
For your wattage range, the XFX and Seasonic PSUs are very good for the money. A Seasonic S12ii 520w is a solid PSU for the $50 range.
Corssair CX are junk, and the less expensive EVGA are also not that good (W1, B, and G series), the EVGA B2 and G2 are good but those start at 650w.
Evga does have a 550w G2 and GS.