id say go with a 550 watt PSU, or something around there
http://www.newegg.com/Product/Product.aspx?Item=N82E16817339001
That won't cut it for high specs gaming. Look for a quality 500watt + unit. Take a peak at the PSU Recommendation list for further details.
I have that, and mine hasn't crashed yet. You'd be suprised by this unit. Quiet but deadly.
Yeah but the HD3850 isn't that big of a power hog.