If you are willing to overclock then get the 7850. It's a solid card at 1000mhz on the core, a basically guaranteed overclock on the card.
The 650ti boost 2gb is a good alternative, but taking into consideration the overclocks the 7850 does it's the best of your options for performance.
430w quality PSU is fine for the 7850. It consumes ~130watts, there's plenty of power on a 430w for the rest of your system.
AMD Gaming Evolved
HTPC: G3258 4.4ghz | 280x 1150/1725 | 8GB DDR3 | XFX 550w | 54" Plasma | Yamaha RX-V667 | 5.0 Paradigm Sound | Xbox 1 Gamepad *Playing: HL2, Fallout 3 Vegas
A wise man changes his mind, a fool never will. - Spanish Proverb