a 500-550w will serve you well then. just be sure its of a decent quality. just go through a few checks.
1. is it certified with 80+ if it is, then thats 1+ for hte compan since they got it checked and tested to some extent.
2. if possible, find the psu on this list
and look at professional reviews.
3. PSU's go on sale all the time, pick a good one thats on sale and go for it.