Heat dissipation / cooling is affected by both enclosure design and HDD heat generation. You can buy enclosures with cooling fans built in, although they cost more. My own opinion is that these just have one more component that can wear out - in fact, a fan's bearings will wear out faster than any other part in the enclosure. So I prefer ones with no fan. But that also means I prefer something with a clean metal case that fits the HDD a bit snugly so that heat flow from HDD to case and through to the outside air is efficient. Some open slots for slow air flow through the case are good.
How hot the HDD runs is another factor. You can get some of this from performance reviews of particular units. But you also can use two very rough rules of thumb. One is: what is the power consumption? Many units these days are about 10 Watts or less, especially the 'green" units that are designed for lower energy consumption. The other is how its performance is rated: those designed and promoted for really fast performance typically run at faster rates and use significantly more power, so they generate more heat and need more heat removal. If you are buying an enclosure to be connected to your computer via USB2, I suggest you do NOT need a high-performance drive inside. The USB2 interface could not keep up with the data access speeds of the super-performace HDD unit.
Bottom line OPINION from me: the good-performance drives like the WD black line and the comparable Seagate units will be quite all right for heat in a non-fan metal enclosure. Anything on the "green" side will be OK. I cannot give you a good opinion on whether Velociraptors etc. will need extra cooling. Even if you go for an enclosure that uses eSATA for its interface (faster than USB2) (you should have an eSATA interface on your computer to use this), a WD black or comparable will be fine in a decent enclosure.
Where I would want a fan in the enclosure for sure, though, is if I were using it to mount an optical drive which has a motor to spin the disk. That uses MUCH more power than an HDD unit, and hence there's more heat to remove. But that is not your case here.
In my own case I assembled one a few years ago. It is from AZIO with both USB2 ad eSATA interfaces (I use the letter), no fan, an external power supply box in the middle of the cord, and its own on/off switch. Inside I mounted a 500 GB Seagate SATA HDD a couple product generations older than today's, I only use it infrequently for backups and it is off most of the time. But when I use it for many hours, it never gets hot - just moderately warm. So I figure that's enough cooling, but I cannot quantify whether the HDD's lifetime is being affected by those conditions.
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