I highly recommend you to buy a quality PSU. A low quality PSU can become a nightmare.
Two GTX460 in SLI need around 300W for themshelves. Therefore, a quality 600W PSU with four 6-pin PCIe connectors is recommended.
Here you have some good options:
XFX XXX Edition 650W (by Seasonic) For 82$ (shipping included) http://www.newegg.com/Product/Prod
Another good options are the following ones:
ANTEC TruePower New 650W (by Seasonic) (modular)
CORSAIR HX650W (by Seasonic) (modular)
ENERMAX LibertyEcoII (ELT620AWT-ECO II) 620W (modular)
ENERMAX Pro82+ II (EPR625AWT II) 625W
ENERMAX Modu82+ II (EMD625AWT II) 625W (modular)
SAPPHIRE FirePSU 625W (by Enermax) (modular)