The 560ti will perform much, read MUCH, better than then 640. See this link for benchmarks: http://www.hwcompare.com/12842/geforce-gt-640-ddr3-vs-geforce-gtx-560-ti/
You won't have to spend much on a new psu, probably time for a new one anyhow. Think $50.00 to $75.00 for psu.