Hiding cables is just attaching them to the cockpit or tv stand so you can't see them. It gives the "finished" look. Use extensions if needed (tv power for example).
http://www.derekspearedesigns.com/Button-Boxes.html
It literally is a box with fancy buttons and lights. They look awesome, are somewhat cheap, make the experience more immersive (real buttons) and use the empty space on the right of your cockpit.
http://www.youtube.com/watch?v=KEU1HbbXOfQ
A simracing screen is a fancy monitor with various data. They look awesome, are somewhat useful and use the empty left space.
Both only work with PC sims/games, but your pc is right there on the left so you could plug the tv screen to it (and most pc games/sims are "free" as we all know).
All g27 cables are already connected anyway (power, pedals, shifter), which are a major pain in the ass, so the only things you need are a cable to connect the pc to your tv (varies on what your tv and graphics card support) and a very common usb extension cable to connect the pc to your g27 more easily.
Without the usb extension if you want to play pc games then you'd have to stand up, disconnect the g27 usb cable from the ps3, crouch to connect it to the pc and repeat every time.
With that usb extension cable it's just putting one end in the cockpit once (attached to it so it is hidden) and the other always connected to the pc, so instead of repeating all that all your life from now on you simply disconnect the g27 usb cable from the ps3, sit down on your cockpit and connect it to the other end of the extension. Miles easier and "finished".
The button box and screen are always connected to the pc.
edit: Even if you don't buy the button box and screen, I highly recommend you to connect your pc to the g27 (usb extension and all). It won't cost more than $10 ($20 if they rip you off in Switzerland) and you'll have like 10x more games for free.
There are plenty of simulators/games that run on lower end PCs and that are awesome. Good minimum requirement sims are live for speed (LFS. "Free" offline s2. Online requires to buy a license), rfactor (also "free" but with online play, and downloading one that weighs like 5gb or more comes with a lot of content), richard burns rally and gtr2 ("free" too and have superb though hard tutorials).