There is a really good 650 watt on outletpc.com for like 30+ shipping.. It's not a major brand so it's not overpriced. I've used plenty of their power supplies in my builds and not once have I had a problem with them.
There are different versions of the lenovo you have.. but they all look like they'd be able to run a 6770 or 6570 just fine as long as the version you have has a PCI-E x16 slot free. But you will definitely need at least a 500 watt power supply.