I've chose a 650W PSU (which is more than I need right now), cause I plan to upgrade it (in a year or two) with another Radeon HD7870. Also in the near future I want to buy an SSD.
Is it smart or should I just get a Seasonic S12II-520 (520W)? I would greatly appreciate any other suggestions.
Grab the 4670K to place on the Z87 motherboard. The Z87 is an overclocking platform, and the K edition is an unlocked processor. You don't have to overclock, but it is free performance. Also, the K edition is only a small additional expense, and has better resale value, in my opinion.
If you do plan on overclocking, you would need to choose a CPU cooler. That can wait.
650W would be enough for two middle-tier cards, but I don't know if it will leave much headroom. Seasonic are a fantastic manufacturer of PSUs.
I would go with a Corsair PSU and use an AMD based system. AMD systems are good for gaming builds, unless you have enough for an i7. But what is your budget?
Corsair doesn't even make their own PSUs. Seasonic is an OEM, their own one, and one for many other companies, namely, Corsair. They make their own PSUs, but make a lot of other companies their PSUs as well.
Every OEM can make bad PSUs. Seasonic has some fantastic PSUs, especially the Seasonic X 650 KV1, but some shitty ones, too.