One last question though, how can you tell if your PSU will fit in your case?
Btw right now I have a HAF 912 and I am wondering what PSUs fit in it and whether or not they need top or bottom fans. Thanks again
Unless you pick a really odd PSU, any ATX power supply will fit in that case. The best configuration would be a PSU with a fan at the bottom to take in air from under the case through the dust filter, but the case supports "upside down" mounting if you somehow want to use air from inside the case (e.g. case sitting on high pile carpet). It looks like an EXTREMELY long modular power supply may hit the little box thingie but it's removable anyway.
Note if you ever for some reason change to case that has a top-mounted PSU, you'll have to see if the power supply cables interfere with the optical drive. If you stick to bottom-mounted PSU "gaming" cases, the only issue you should be looking at is cable length. (e.g. does your motherboard put that 4-8 pin CPU connector all the way at the top-left?)
"The computer programmer says they should drive the car around the block and see if the tire fixes itself." [src]