>>346
Well if you've never come across って before you're not being an idiot, and if you have I'll forgive you.
って is a casual version of と or という or variations thereof. It gets used all the time in many different ways, often as a handy way of talking about or descrbing whatever it's "quoting", which can be pretty much anything.
Here's it's talking about the word そんなに itself, which presumably someone else said before - I would guess something along the the lines of なんでそんなにｘｘｘ? or そんなにｘｘｘなくてもいい"? If so (and if not you should be able to understand from this example anyway) your sentence means something like "What do you mean 'so xxx'?!". It's just questioning why the other person considers xxx to be at a level worthy of the "そんなに".
>>347
It doesn't really mean anything. Often katakana gets used for slangy words, e.g. メシ is much rougher and more causal than ごはん and ホント correctly is ほんとう with the long vowel. Also words with difficult or rare kanji sometimes get written in katakana, especially in things aimed at a teenage audience. (In fact overall this kind of katakana use is most common in shounen, in my experience.) Lastly katakana is sometimes used to show that a word is being said with a lot of emphasis.