well, a decent 900W supply is ok. a $500 computer isn't going to strain it
What we'l like is the amperage notes that are on the side of the psu, and probly repeated on the sides.
It should lok something like this (the numbers are from an Enermax 90W 80+ Gold PSU. yours probly will be different)
+3.3V@24A, +5V@24A, +12V1@30A, +12V2@30A, +12V3@30A, +12V4@30A, -12V@0.5A, +5VSB@3.0A
Also when did you buy it. a $500 computer is going to be pulling 200-300W which will lower the efficency of a 900W PSU (as its only 22-33% of max, best efficiency is 70-85% of power)