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It's all compatible, though the PSU isn't the best choice. Rosewill's units are just ok. You'd do better to get a smaller, high quality PSU. I'd look at the XFX 650W, which is $60 after rebate. It's the same total price, but a lot higher quality. You could even drop to a 450-550W PSU from another quality brand like Antec (their Earthwatts series only), Corsair, SeaSonic or Silverstone, since you're not going to be able to SLI (the board doesn't support it. However, most of them are either sold out right now or more expensive than the XFX 650W.
Kind of. You need to make sure you have an ATX case with at least enough room for the GPU. You want to have at least two fans (one front, one back) for ventilation. Other than that, it doesn't matter too much. Cheap cases tend to be really flimsy and hard to build in. That Challenger case is easily among the cheapest quality cases you can buy. You could go cheaper, but more low-end cases have a big shpping charge. Typically, the least you can spend on a case is $40 with shipping. The extra $15 for the Challenger would be well worth it.
That's not a good idea. The non-"K" versions of the Sandy Bridge CPUs aren't able to be overclocked. That means getting a HSF with it is pretty much pointless. As is buying a P67, as the main benefit of the P67 is that you can overclock.
If you really want to get a CPU that you can't overclock, switch the board to one that won't let you either. The H67 series boards will provide you with all the other features that P67 don't, such as integrated graphics and Quick Snyc.
That's also not the best HSF either. This combo, the 2500K and Coolermaster Hyper 212 Plus, would not only allow you to overclock, but gives you a better deal ($8 off vs. $5). The Hyper 212 is also the best budget HSF out there, and is the one I'd recommend at $30.
You might be able to overclock it a little, but it wouldn't be easy or recommended. It's restrictive enough that if you wanted to do any overclocking, the extra $30 for the 2500K is well worth the money.