The USB 2 Adapters are not as reliable and do not perform as well (bandwidth) as the PCI slot variety. I have both types.
Yes the rest of the components look good.
Your motherboard will support 8 GBs per slot - this is something to consider. My suggestion is to go for two 8 GB Corsair Vengeance modules for a total of 16 GBs. - Do you have any memory upgrade planned for the future?
This is my first time attempting to build and game on a PC so I haven't put much thought into future upgrades yet. I know for sure I will upgrade some parts eventually though.
But from what I've read 8Gb (2x4Gb) seems to be plenty. Would the benefit be worth the extra cost?
Yes 8 GBs will work for you.
In case you are planning to populate all four slots then the module (tall Vengeance RAM) in the slot closest to the CPU socket will interfere with the heatsink fan. I have four 8 GB modules of Corsair Vengeance )tall) RAM and the Hyper 212 EVO heatsink. The way I got around the interference problem is by disc grinding off 2mm from the bottom portion of the heatsink fan. In your case since you are using only two Vengeance modules you will not have any interference issues resulting from the tall RAM.
It won't help with actual gaming fps improvements, but your games/programs will load faster and your computer will boot up more quickly. In my opinion, it's a worthwhile addition once you get the chance, but it depends on your use I guess.