It really depends on what you can do with the card. Eater of Days is a better card, so I think, and especially is if you can counter the triggered ability with something like Stifle. Lose no turns, HA!
Anyways, if you have more of a use for Eater, then get it. Otherwise, don't. As you say, it's risky, and without a use, probably not worth it.
Also, the XX means you have to pay the same amount for both X. So, if you pay 6 mana for it, you get 3 charge counters (3 for each X).
And I'll go with your little sig and explain limited real quick. Limited just means you play with only the cards you open, except basic lands. For example, a booster draft is a Limited event because you can only play with the cards you draft, and then can add any number of basic lands. Same for sealed deck.
As for staples, there are no staples per se. However, there are really good cards. For example, Umezawa's Jitte. Great card. Fits in a lot of decks. Not in all decks however.
The closest thing to staples would be the really good cards. They aren't required for decks, but without them your chances of winning a mirror match with them go down. Again, back to Jitte. You can have a nice White Weenie without the Jitte, but most of the time, if you play against a White Weenie that utelizes Jitte, you have less of a chance of winning.
The better cards vary from deck-type to deck-type. Your best bet would be to look up decks on StarCityGames or on the official Magic: the Gathering website.
Anything else needs to be cleared up, feel free to post and/or PM me. You could also probably PM the other MTG players, but I'm not sure if they want that or not, so I will not speak for them as such.