The HD 7850 under load pulls a max of ~100 watts using Furmark. It will never pull that in any other situation.http://www.techpowerup.com/reviews/AMD/HD_7850_HD_7870/24.html
The Corsair 430CX is made by CWT and is an overall great power supply. There are likely 2 versions on the market right now the 430CX V2 and the 430CX V3 that is replacing it. The V2 is 80 Plus Certified and supplies 28 amps on the 12v rail. The new V3 is 80 Plus Bronze Certified and supplies 32 amps on the 12v rail. Both versions are sufficient for a system with an HD 7850.
If you plan on running 2 cards in Crossfire go with a 600w PSU.