Really depends on your budget. I would try to get at least the 660 for that some gaming and make it a more reliable brand like eVGA or Asus. The extra horsepower does help in Photoshop too. My 580 helps a lot when I'm touching stuff up and makes everything real snippy.
I would keep everything you have currently besides change the RAM. You really don't need 2400 RAM. Anything over 1600 isn't noticed and beyond that is a waste of money.
You can save on cost by swapping the Z77 board out to an H77 or B75. There isn't a reason to have the Z77 when you wont be overclocking due to not having a K processor.
You will also need a PSU too.
Either of these will do fine:
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