What size power supply should i get?

Hi, Im going to build my first computer this christmas, and have been getting parts throughout all these black friday sales and such, and now I'm down to just the power supply. Here are my specs:
8 Gigabytes of memory (2x4) 1600 MHz
Biostar tz77 motherboard
Intel i5 3570K
23 inch 1920x1080 moniter
gigabyte gtx 660 ti OC
and a nzxt phantom case
I was thinking a 600 W power supply because i also plan on overclocking in the future (not immediately, i will probably do it later when i can get a better cooler) so is 600W right or can i accept a lower power power supply?