The most important thing in case cooling is airflow. The size of the fans is meaningless if the air just pools in dead spots. Also the quality of the fan is important. The cheap ones are cheap (like under $5), but they are usually the really loud ones. When you're talking the good ones, the difference in loudness isn't really all that noticeable, especially from a distance. Actually, my old computer has seven (yes seven!) 80mm fans.
When I built a new computer for my dad a few months ago, I was going to reuse the case. But I would have taken everything out, clean it, etc. and he wound up getting one from a guy at work for free. It had three 120mm fans. In the end, it actually was louder than mine was.
Also, the difference in air movement isn't really all that much. You can hold a 80 and a 120 up to your face and see for yourself. Is it there? Yes. Is it noticeable? Not really. And in the end, probably only the difference of 1 or 2 degrees Celsius of ambient temperature.
The cost for quality 80mm fans will run about $10 a piece whereas a quality 120 goes for $15-$20. The difference in price would be better severed going towards a better case. Now if you have the cash to spare, by all means get the bigger fans. Or even just buying one for the exhaust in back is good. As long as there isn't much getting in the way (a ball of wires, four hard drives, etc) 80mm is plenty sufficient for the front intake.
Actually, most cases nowadays come with the option of 2 80mm or 1 120mm fan for the intake and most people choose the two 80mm.
When looking for a case, try and find one that has good wire-routing options. The good ones these days have rubber holes that you can use to route the wires under the motherboard try and back out into the drive carriage. It should also have plenty of easily accessible zip-tie hooks. for grouping wires and tying them down out of the way.
The drive carriage itself should be nice an neat and have some kind of option to easily get all your drives together up and out of the way.
You can also invest in a PSU that has modular wiring--that is having outlets for plugging in only the wires you need. This save you from the inevitable clump of wires that tends to get in the way of everything.
Your ultimate objective is to create a nice clean environment with no obstructions to airflow.
It is possible to achieve the same goal with cheap stuff. I built one for my mom around the same time as my dad's. It was much less robust and the total cost of the case and PSU was under $50. Running the wires was much more painstaking and extremely time consuming, but after 2 hours, a lot of trial and error, I was able to make use of ever nook and cranny I could find. I was still able to keep the ambient around 45C under load.
That's a good number to shoot for. 50 is considered "normal" and 40 is optimal. But as long as your GPU has its own cooling and the memory heat spreaders (and I don't think it's possible to buy sticks without them these days) you should be totally fine.
Also, remember room temperature plays a HUGE part in the case temp, a lot more than most people realize. I know it's trendy for people to put their family computers in their sun rooms or solariums. To me, this is just blatant stupidity.
Now CPU cooling is a horse of a different color. It is much more important and every single degree matters. IMO a good aftermarket CPU cooler is the most important investment when building a new computer. The stock ones are fine for normal office work, but if you plan on doing any multimedia at all (especially gaming) buy one.
If you really want to go crazy you can get one of the hydro-hybrids. They use a radiator/fan combo. That way you get the benefits of hydro cooling without having to worry about installing pumps and hoses and all that. I think they run about $80. I've never used one myself, but a friend just did and she said it wasn't too difficult.
For standard air cooling, the Coolermaster Hyper 212 is current champ. It's not hard to install as long as you follow directions (and there are dozens of YT vids) and it cools like a boss.
If you get one, keep in mind that it's pretty big so double check dimensions of stuff. Though it shouldn't be a problem.
Here, your goal should be 70-75C under load. 80 is considered normal and 90 "okay" and usually 100 is the danger zone. Though with a little work, people can achieve 60 under load which is certainly optimal and will greatly extend the life of the CPU.
