I am building a PC mainly to play SWTOR. I really wanted to GTX 570 but I like the price of the 560 ti. The difference between the two is hard to justify the nearly $100 difference. I saw the superclocked version of the 560ti at Amazon for $250.
Performance difference on a game like SWTOR will not be noticeable. Most MMO's of that calibur will be more CPU heavy, so make sure you have a good CPU that will not bottleneck the 560ti. Anything modern with four cores at about 3.5ghz will do just fine.
As for dangers.. really, there aren't any that would necessarily damage your system. An overclock simply reduces the overall lifespan of whatever it is that you are overclocking. Most cpu/gpu's lifespans are like 10 years before they die on their own. Overclocking might reduce it like.. 8.5 years at the most? But by that time, you will want a new card anyways so it doesn't matter. As for the bad things that could go wrong with a superoverclocked card.. well for one, the card itself could be unstable (unlikely) to the point it just won't work right. It won't damage anything, it just wont work.
Honestly though, I would go with this card instead and buy it from Newegg: http://www.newegg.com/Product/Product.aspx?Item=N82E168.... Its overclocked (and can be overclocked further), it has a better cooling system and it will be much quieter than the card that you linked. Not to mention its cheaper with a mail in rebate. Only thing you don't get, I guess, is a copy of the batman game, which must have JUST ended on newegg.