The link above gives a brief outline of the difference between the 2. I'm probably not going to do crazy overclocking, maybe up to 4.5 GHz. So I am not sure if I really need the extra power phases in the Pro? Besides this, and the extra USB 3.0, I don't see much difference between the P8Z77-V and the P8Z77-V Pro that justifies the extra cost.
PSU: Seasonic M12II-520W/620W
I am pretty sure that with my current build, 520W is pretty ample. But should I get 620W for some headroom in case I'd go for Crossfire/SLI?
for sli/xfire i would feel more comfortable with a 700-800 watt supply. change the Ram to a low profile one or just anything else besides Corsair vengeance as it has really high heatsinks that will block the hyper 212 evo
The power supply size is determined by the graphics configuration.
Today's 28nm graphics cards do not need more than a 500w psu. 650w tops.
A GTX680 or 7970 will run on 500W.
A 650w unit is close enough in price to a 500w unit and will run cooler and quieter to warrant a slight premium.
Whatever you do, buy a QUALITY PSU. A cheap unit will be underpowered, and dangerous to your other components if it fails.
My short list of quality psu's would include Seasonic, Antec, Corsair, PC P&C, and XFX to name a few.
With the availability of cards as strong as a GTX690, I see no need to spend extra up front planning on sli.
For gaming, it is the graphics card that will give you the FPS you want, not so much the CPU.
Your 4850 is not going to give you the performance that a ivy 3570K is capable of.
If a big graphics upgrade is in order, fine.
Any Z77 motherboard will give you an OC to 4.0 and higher. That is all you need. Even at stock, the 3570K will drive most graphics cards to it's limits.
What I am saying is save your funds for a better graphics card.
Consider a cheaper Z77 M-ATX motherboard. It has only 4 expansion slots, but how many of the 7 on a full ATX motherboard do you plan to use anyway?
With a Monitor Resolution of 1280x1024 SLI/Crossfire shouldn't be needed. I would upgrade from a HD4850 though. A single geforce 560 ti (or equivalent ati card) should easily make any game playable at max settings.