1. Ah, they use small cases, forget about that NH-U12S.
2. Load = stress, f.e. if you play game then cpu is under load, so when it`s overclocked ... and then - all cores are stressed - it needs a lot of power. But first - decent cpu cooler for overclocking and ... it has to fit in your case.
Maybe something like this:
http://www.komplett.dk[...]
if you have hole in that case for 120mm fan (probably on the back of the case).
3. You should buy it asap for that price (1699kr.), you can always use it from ... today / tomorrow ... etc. - it`s a separate component.
4. Well ... you can always install games on the one that you have and buy HDD - as a "lager disk" (movies, music, data), f.e.:
http://www.edbpriser.dk[...]
- it`s a silent one.
Up to you.
--
P183 V3 @ 500rpm, P8P67 Deluxe, i7-2600K @ 4.0, NH-D14 @ 500rpm, 840 PRO 256GB, 8GB 1866/CL8, GTX 580 + Alpenföhn Peter @ 500rpm, SF 1000W Platinum