I don't think that SteelSeries 7G is worthless, especially if you like the "linear switch" touch, although in Japan it's only "slightly above average" keyboard thanks to the transit expenses.
(And same applies to Filco keyboards outside Japan, I guess)
Well, back on to the topic.
Most PC keyboards are just 2-key rollover capable, which means you can resister 2 key inputs at the same time without problem, regardless of combination of the key.
And of course you can resister certain key inputs regardless of specifications on paper, like all PC keyboards should have no problem with ctrl+alt+del.
But advertising for cheaper keyboards ends here; when you play some computer game, there's good chance that the key combinations you want just don't work. Like, you press up and right arrow keys to move, space bar to open fire, and then press "t" for targeting and it doesn't.
If you are a good touch typer you may want a 3-key rollover to avoid "jamming" but the incapability of the keyboard should not be a major problem.
If your typing is extremely fast, you probably need to resister 3-4 key inputs at the same time.
If you are an FPS gamer, you may need 5 or 6 simultaneous key inputs.
And, if you are a dedicated and serious MMO player (World of Warcraft, for instance) you may need 8 or 9 keys at the same time, where most USB keyboards fail, though you can still use modifier keys to 7 or 8 inputs.
If you need to input Braille with your keyboard, 6-key rollover is sufficient, but actually it needs only one fixed combination and non n-key rollover keyboards have good chance to be Braille-capable (like Microsoft Digital Media Pro Keyboard, which is no good on games).
I personally recommend n-key rollover ones because they have apparent superiority on functionality, but if you are just average touch typer who don't play games and have no business with Braille or whatsoever, then it may be unwise to pay additional cost for uncalled extras.
