Most cards come with an adaptor or adaptors in the box, there is no need to buy one seperately-but check before making the purchase
Only the more expensive PSUs' split their output, in most cases you'll see the wires are all soldered to a single output for each voltage.
Even a very poor 700W unit should handle the 5770 with no problems, even if you are using a molex/PCI-E converter.
This is a handy tool to calculate system power needs:http://extreme.outervision.com/psucalculatorlite.jsp
If you are considering a new PSU: Corsair, OCZ, Antec, Seasonic and FSP are the cream of the bunch.
Every e-tailer has their own 'specials'- so shop around but this looks good:http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016
Modular and with plenty of power, it's quite cheap too
Unless, like me you do not live in the USA.