The disk drives absolutely do generate a lot of heat and you probably do need active cooling on them.
If the drives get too hot [or too cold], the life is shortened and they may fail. If they get really too hot, they may fail quickly (e.g. after two months instead of two years). The ideal range is 25 degrees (C) to 40 degrees (C). See http://www.buildcomputers.net/hdd-temperature.html
Most SATA drives can report temperature as part of their SMART data. Just get a utility that displays this. With the right one, it could be set up to monitor continuously and post a warning if a drive gets above/below a threshold
In my system, I have venting holes on the case front with a fan drawing air inward toward the back directly across the drives. That is, the fan is directly behind the case front, blowing on the drives which are 1/4 inch away from the fan. When this fan failed [I got lazy and didn't replace it on my normal 6 month rotation], the drives overheated and 2 of the 3 became flaky, died, and had to be replaced.
Note that my case had the usual exhaust fan at the back, and an additional fan on one side. They weren't enough to protect the disks because they didn't provide the airflow to the drives directly.
So, I do recommend active drive cooling--YMMV
UPDATE:
Ok, your disk temp is within range, but that's light loading.
If you want to go the extra distance, because you've got a brand new system, I'd stress it a bit at maximum load and remeasure. Don't know if you're using WinX or Linux/BSD on your system, but you can probably find a program that does just that.
This will get all CPU cores going at 100% and will continuously copy data back and forth between your disks into junk/temp directories that you can delete later. Run it overnight, with the temperature program logging [say] every five minutes. That way, you're assured.
In the good old days, most vendors would burn in a system for five days before shipping. That's because of "infant mortality"--a chip will usually fail early, or last 20 years. Today, you're lucky if you get five minutes or five hours of burn in before a vendor ships :-).
So, if you run the above test for five days [you can pause it if you want to watch a video], you'll be doing your own QA. That way, you can have total confidence in your system before you start loading it up with video files, etc. IMO, well worth it.
This might help detect hot spots, if your system has any. Run the test. Then power off the system. Probe around inside and see if anything seems warm/hot to the touch [usual caveats about static electricity apply]. CPU will be fine with a properly installed CPU cooler, so no need to probe that.
What about airflow across the motherboard and RAM chips?
A number of cases these days come with the drive bay fan built in. But, even if you don't want to go for that, I'd probably add at least one fan, and not rely on ambient. The standard place is the exhaust fan at the back. Each case design has its own place to put this.
Power supply fans are designed to keep just the power supply cool. Switching supplies generate a fair amount of heat. Their fans are not intended to function as the primary exhaust fan of the system. It puts an additional load on them that they're not designed for.
For example, I just measured my system:
Room Ambient: 25.0 C Exhaust: 29.5 C Power Supply: 35.8 C
Replacing a case fan is relatively easy. Replacing a power supply fan is more difficult [if you can even find a replacement fan]. Also, consider the dust factor. That's what kills most fans. So, periodic cleaning, but sometimes, even with cleaning, you're better off with replacement. Having a separate exhaust fan will reduce the amount of dust/gunk that gets vectored through the supply.
A fan is $10 and protecting a >$1000 investment.
Also, it is fall. On my system, I might be able to reduce the fans, but in summer, that's a different story.
BTW, if you do decide to add one, I like Enermax fans. I've got the 3-speed adjustable ones. Enermax has just about the highest CFM I've been able to find. And, despite the yak-yak about fan noise, they're just as quiet as the so called "quiet" fans [which probably advertise "quiet" because they can't match the CFM]