I need assistance choosing a PSU, I'm deciding between a corsair HX850 or an HX750. Normally I would go for the 850 just to be sure, but at this moment in time the HX750 is 60$ less. Should I save some and go for the 750 watt or stay safe with the 850 watt?
I'd also like to note I would like to add another 780 ti graphics card in the future and maybe several other HDD.
Either is fine and is overkill. If you have at least a good power supply from 550W and upwards it will be alright for single gpu setup.
But i have to ask, what are you using your pc for to use 32 GB RAM, an i7 and the need for the sound card?
I plan on using this as a temporary gaming machine and later turn in into a server/work machine. I am also not going to get the sound card or 32 GB right now, but I left it there to simulate what the PSU needs to handle.
I'd say neither. If this is your full system setup, something like a 620/650W gives you loads of headroom, so 750 vs 850W is kinda pointless.
If you are going for SLI GTX780Tis, then you would have to ditch the soundcard anyway.
Also, the HX750 was on offer, which is why it was so much cheaper than the HX850, but assuming you are looking at newegg prices, it now shows as unavailable.
If you want something to power that system at a sensible price, I'd suggest the Antec HCG-620M. It's been on offer a lot recently, and even without should be sensibly priced. See also the NXZT Hale 82 650M, Seasonc M12II 620/650.
If you want fully modular and/or higher efficiency rating, I'd suggest considering the Rosewill Tachyon 650. 80Plus Platinum, fully modular, a great OEM and under $100.
I thought the sound card would be able to fit in between the two graphics cards. If not, it is not a deal breaker but it would be nice to use the PCI express x1 slot on the MOBO.
Not on that motherboard, you have 4 slots to play with, and each graphics card takes up two (overhanging the slot below it).
In some mATX case/motherboard combos, you might have space, but you'd need a case with 5 expansion slots and a motherboard with a suitable layout option.
As an example - Two of the most popular mATX cases for high end builds are the Corsair 350D and Fractal Design Arc Mini - the Corsair has 5 expansion slots but the Fractal only has the 4 the standard requires.
For a motherboard, the only one I think would work would be the Gigabyte G1 Sniper M5, though I'd have to double check.