I would concentrate on getting a good core for your machine. By that, I mean CPU, mobo, and RAM. get the 4790, 16GB and a compatible mobo. You don’t need to go overboard on this, just get one that has 4 RAM slots, SATA3, a decent onboard sound-chip. Basicaly, anything from a big mobo name. (I’m not a great believer in expensive mobos).
If you are stuggling to stay within budget, here are some good options.
Although I prefer Nividia, the AMD 290 or 290X cards are great value. The GPU is usually the first thing you upgrade anyway.
You can get a cheaper HDD for now, and get yourself an SSD later.
A sound card is another thing that you could add later.
PCI slots are for things like sound cards, wifi cards, etc. All those add-ons that don’t go in your PCI-E slots (which is basically where your GPU goes).
Do NOT skimp on your PSU!
Good luck! 