Video cards only go into PCIe x16 slots, the sound card doesn't matter which PCIe slot you put it in, even if you blew a full sized PCI-E slot, you could put it in there.
PCI-E 2.x and 3.x slots are identical, they just handle signaling differently according to the card that is connected. It won't matter, other than say putting the video card into the PCI-E 3.0 slot(s) for maximum performance.
and good lord, a $300 motherboard. Really? Yeesh. Nothing wrong with it, but your pocketbook is the size of the Grand Canyon. o.O
Hard drive is a little excessive, be sure the motherboard can boot from it. You might be happier with an SSD/HDD combo (one of each) or a lower sized HDD. But hey, the choice is fine as long as the board is OK with it.
RAM is fine. Heatsink is good if you're planning on overclocking, otherwise no need to buy it
Power Supply is fine
Video Card is uber aka fine
You did fine overall, if not a pricey build.