i'm working on a heatsink buyer's guide right now at amd.com's forums, but here's a brief:
Types of coolers:
The best air coolers right now use HDT (heat-pipe direct touch) with 4-6 pipes that rise into a fin-studded tower about 160 mm tall. The exception is the Thermalrights (such as the True and Ultra) which have a base between the heatpipes and the cpu, but will beat HDT coolers in some situations. (more on that later) but are close enough to be comparable.
If you are looking for a non-air cooler, the ones costing less than $100 are nowhere near the performance of HDT.
Designs:
There are two main designs for HDT coolers: those with the fins tower perpendicular to the motherboard and those with the fin tower parallel to the motherboard. the way heat-pipes work is that there is a heat-transfer liquid inside the pipes that evaporates when heated, rises into the fin tower, condenses, then returns to the heated area. It appears that the excessive bends of the parallel design are interfering with the condensation/return of the fluid inside the heat pipes, and parallel designs are not performing nearly as well as their perpendicular counterparts.
more information:
www.http://en.wikipedia.org/wiki/Computer_cooling#Heat_...
Height:
Right now, most of the top HDT coolers have multiple versions. One alternate version usually offered is the same basic design, but a slightly shorter fin tower for ease of installation. In most cases, the cooling performance is reduced by 10-25%. Imho, one should get the 150mm tower and remove obstructions, e.g. a side case fan. Example: Xigmatek S1283 vs S964.
Fans:
Not all fans are created equal. All fans are rated by CFM, or cubic feet per minute. Silenx fans are rated the highest, and should be considered when buying a cooler that does not include a fan. As a general rule, the larger fan has a higher CFM, and between two fans of the same size, larger blades equals more CFM. This is one possible reason why there's such a large difference between the two identical coolers but one is slightly smaller. Also, some coolers allow for 2 or more fans to be mounted. In most tests, this results in about a 1C difference in performance, which is negligible.
Example: The Xigmatek S123, S124, and S124EE
www.frostytech.com
Contact:
When the hsf meets the cpu, the amount of contact area determines it's cooling effectiveness (which is one reason why you'll see completely different results for the same cooler across two different review sites.) Several issues impact the amount of contact between the two:
1. Pressure: In a study done at Innovation Cooling, it was found that insufficient pressure prevents the hsf from making good contact with the cpu. if you are using an Intel mounting plate, this is much easier to deal with with washers, but much more difficult to deal with using the AM2 clips. This is possibly one reason why the Thermalrights perform so well even without HDT: they only come with backplate mounting, even for AM2 boards.
Source:
http://www.innovationcooling.com/Indepandentuserpressur...
2. Flatness: This is where the whole issue of lapping comes in. Some hsf's do not (or can not) make good contact with the cpu because one or both are not flat. thermalright coolers usually have bases that are concave to fit Intel processors that are usually convex. this was done purposely by thermalright. If you do not wish to lap your Intel cpu, then by all means get a thermalright. if you have an amd cpu, then you should seriously consider lapping the thermalright. however, in theory, the pressure maintained by the backplate mounting system should overcome this slightly.
Source:
http://www.tomshardware.com/forum/235216-29-lapped-ultr...
3. TIM (Thermal Interface Material): For any machined surface, there will always be microscopic scratches and imperfections. Therefore, TIM is used to fill these spaces and improve the contact area. What is important to understand is that not all TIM's are created equal, but under certain testing conditions, two different TIM's can vary by -5 to +5 on the same setup. For the average user, correct application is far more important than brand or material used in the TIM.
Best rated brands: Artic Silver 5 or Ceramique, Masscool Shin-Etsu, IC Diamond 7, Artic Cooling MX-2.
TIM Application Guide:
http://www.innovationcooling.com/applicationisnstructio...
Noise:
Nearly all websites rate coolers by noise nowadays. The basic finding is: nearly all coolers are noisy when the fan is at full speed, and much quieter when the fan is at a lower speed. Therefore, it is impossible to find a well-performing, quiet, cooler. However, it is much easier to find a cooler with a fan control, preferably as an external, easily reachable option. Also, it is possible to get fan noise reducers, like vibration controlling rubber mounts. Purchased separately, an external fan control is about $6-10.
Voltage:
All coolers perform differently depending on how much heat they are required to dissipate. This is one reason why an "idle" measurement for a cooler is nearly useless.
www.frostytech.com is one of the only websites that use a triple-watt measuring system, and rate coolers across 3 different loads: 85 watt, 125 watt, and 150 watt.
Cost:
As a general rule, Thermalright and Zalman are twice the cost of Silenx, Coolbeam, and Xigmatek for essentially the same design and performance.
Performance:
Today, the top 5 or so coolers are within 1C of each other in terms of performance, which is not a surprise as they all have nearly identical designs (and which is why non-HDT coolers were not discussed in this article.)
Installation:
As a general rule, all hsf's that use backplate mounting must be installed with the mobo removed from the case. Also, all hsf's that use AM2 clips can be mounted on AMD boards without removing the motherboard, but intel mobos still need to be removed.
Recommendations:
1. Coolbeamtech Core Contact Freezer ($35-40): comes with a PCI fan control, AM2 mount.
2. Silenx iXtrema IXC-120HA2 ($40-45, $6 more for extra fan): Can have a dual fan mount, but you have to buy it separately. AM2 mount. External fan control, hangs inside the case or can be mounted outside the case with double-sided tape.
3. Xigmatek S1283 ($35-40): Snagged Tom's "Best of 2008" award, but the two hsf's above were not reviewed. Note: the s1284 has the same heatsink, but a worse fan and worse performance. It was updated with a larger fan in the s1284EE model, which does outperform the s1283. AM2 mount.
4. Thermalright True or Ultra ($70-80): no option for AM2 clips, must be mounted with backplate. Probably a better match overall for Intel chips. Personally, I don't like the base design, because this is the only one that beats an HDT design. All other coolers with similar bases have far worse performance.
5. Zalman CNPS 8700: if height is a concern and none of the others above will fit, the 8700 is one of the few high-performing HDT designs with a lower profile (66mm.)
6. Thermalright IFX-14: ($100+): features quadruple possible fan mounting, three fin towers, and a backplate heatsink for the CPU...if only multiple fans worked! Hard to find.
Other types of cooling solutions:
Right now, at the cost and performance, HDT can't be beat. However, if you are willing to spend a little more:
All-in-one water cooling:
Don't. HDT>current home-use water coolers under $100. A good water cooling system will generally run $200+. The best non-hobbyist package right now for the home atm is the Zalman Reservator XT.
Hybrid-Peltier:
There's only one company on the market at the moment, Coolit. Their coolers are $140-300 and perform like air conditioning. However, they still have issues with condensation, and their current model, the Pure, does not have a distributor.