The first thing to ask, is are you intending to use TWO graphics cards? If you are, then obviously a high rated PSU will be required. If not, then there is no point to using a Z77 motherboard, with a locked processor. If you only intend one GPU and want to use a locked processor, then use a lower cost H77 mobo.
If you want to use Z77, get an unlocked i5-3570K, to at least, give yourself the option of overclocking, in the future, even if you don't necessarily want to now.
Having said that, with a locked processor, and SINGLE HD7870, 500w or slightly above is adequate. It depends, of course, where you are, but the XFX 550w is a good PSU, and normally competatively priced. 550w gives scope for GPU overclocking, and modest additions/upgrades. If you want scope for adding 2nd GPU, you really want 750w. At that rating, there are lots of decent choices; XFX again, Corsair, PC Power&Cooling, Antec, Seasonic.