You'd actually could spend MORE money with a smaller power supply. Your actual power consumption with what you have described and a 660 Ti will be just under 300 wattshttp://www.plugloadsolutions.com/psu_reports/SP508_SILVERSTONE_SST-ST60EF_ATX12V_600W_Report.pdf
This shows that at a 300 Watt load, your efficiency is at 86%
Dropping to a 450 watter, your efficiency drops to 84%http://www.plugloadsolutions.com/psu_reports/SP341_SILVERSTONE_SST-ST45NF_450W_Report.pdf
The 450 watter will provide the same 300 watts but will do it with a 2% efficiency penalty and therefore consume more electricity.
At idle, that box should draw about 120 watts which gives an efficiency of about 83% for the 600 watter and 83% for the 450 watter.