Time to carry on, although not much more assembling needs to happen.
First and foremost: the power cable
I was looking for a 6-pin PCI-E power connector. Or adapter. Or 6-pin MOLEX adapter. Whatever, it looks like this:
You plug two HDD-power connectors into the 4-pin sockets, and plug the 6-pin socket into the graphics card. Boom! Power.
The bad news is that I don't have any more 4-pin power connectors, so there is no way to install an optical drive right now. Oh well.
Wireless capabilities
I first thought of installing a wireless network card; but of course, the graphics card is so huge that it would not fit. I then researched an external wireless adapter; sure enough, there are some USB-based adapters out there. It's just tough to find a good one, with good enough signal reception, and one that does not just break after a couple months of usage. So for now, I'm OK with a LAN cable stretched across the house.
Sound
I already have headphones; they're comfortable and whatnot. However, I frequently listed to audio books while doing chores around the house, as well as when painting and assembling miniatures. I don't do all of that in front of the computer. Previously, I just moved my laptop around the house; however, that is no longer an option.
Option a. Wireless headphones
I went ahead and bought this: http://sumvision.com/product/akuma/akuma.html
It's actually a pretty sweet deal. All reviewers agree that it's a great product in its price range.
I can only add that it seems uncomfortable when putting it on at first, but I got used to it pretty fast. I don't even notice it any longer; I can walk around the house, do stuff, or even sleep, with this thing on.
A second thing to add is that its wireless capabilities are great. I can walk around with only minor disturbances from time to time, and it gets across multiple rooms. Sadly, it does not work if I'm out in the kitchen painting. I'm guessing that the intervening bathroom and the multiple tiled walls are to blame. Nevertheless, I'm happy with my purchase.
Option b. Speakers
Now I would have been happy with a simple set of desktop speakers and be done with this, but wifey needs her movies (and I have to admit, watching them on a huge monitor is great - why not enhance the experience with quality sound?), so we decided on something bigger.
My first line of research went into 5.1 surround sound systems. It's already hard enough to find a good one; the market is flooded with products, and reviews are sparse. Fortunately, we have a friend who knows his sound systems. I quickly found out that quality costs a lot (just look at JBL, Altec Lansing or Harman Kardon). Moreover, these things are designed for home cinemas and smart tvs. They don't interact well with a PC, unless you have a dedicated soundboard.
My second line of research went into so called "gaming surround sound systems". Simply put, these are 5.1 systems designed for PCs. The advantage over the other kind of sound systems is the simple 3.5 audio jack input. So I went for this: http://www.logitech.com/en-ca/product/speaker-system-z906 So far, I haven't used it much. Right now, my computer is right next to the couch, and there's no way I can create a surround-setup for the speakers. I'll relocate it to the opposite corner of the room soon, and then we'll be able to watch movies comfortably from the couch. I can say that those few times that I did try out the speakers, they sounded great, and the bass is nice, too.
Epilogue
As any fan knows, a PC is never complete. You can always buy a new accessory, or improve an existing component. I may do so in the future; for now, I'm quite happy with what I've assembled.
Happy gaming!