Intel CPU's doubt

Can't really tell until benchmarks for the i5-3450 is released, but yes, your computer is compatible.  

And when will benchmarks for the i5-3450 get released?

When the chip is released, so maybe in mid-May?

But the chip has already been released, because I see it on sale in stores here in my country.

Wrong country I was looking at. Sorry. It should be released in a week or so then.

Should I opt for the mobo I posted initially or for this one? Asrock Z77 Pro4

If you do not plan to OC, get the H77 mobos when they release.

And if I want to install one more GPU in the future?

H77 should have 2 slots on their mid-end boards.

But my asrock Z77 Pro4 already has 2 slots.

I would get the i5-3570k then. I see no point in a Z77 MOBO if you get a CPU that can't overclock.

You're right. I won't opt neither for an i5-3570k neither a Z77 MOBO. I will just opt for a H77 MOBO and the i5-3450.
Tell me which H77 MOBO should I choose to have one more GPU in the future if I want to.
The site is this one - kuantokusta.pt
You may have to translate it to portuguese.

I'm building a rig myself for the girlfriend right now, and I went with a I5 3450 as well (since i'm not planning to OC this build).

The asrock aren't a bad option, even if you get a Z77 board like the pro4 or a extreme4. They aren't that much more expensive then their H77 asrock counterparts, especially if you get a deal on them. And who knows, maybe you would want to swap out for a k series cpu down the road...if haswell is as good as rumours say  

For my build, I went with a ASUS P8H77-V mobo, it doesn't have two PCIe 3.0 slots and is pretty good bang for buck if you aren't going to crossfire the card anytime soon...