Winbond UTT CH-5
The claim to fame of the Winbond chips is their ability to maintain very tight timings at high clocks. However, these chips are strongly not recommended for people that do not intend on overclocking their RAM. Those of you looking to have some fast memory at stock speeds should look elsewhere. Sure, the CH-5 will be fast at stock, but it needs very high voltage at stock to be as fast as TCCD is at stock on low volts. For instance, look at the OCZ VX series of RAM, which uses exclusively Winbond CH-5. The stock voltage on those chips is 3.3V. Nevertheless, CH-5's ability to reach fairly high clock speeds at insanely tight timings is very appealing to heavy overclockers (especially AMD overclockers), and is probably the main reason you can rarely find it for sale.

Winbond UTT BH-5
Ahh, the elusive and sought after BH-5. When Winbond released this chip after its BH-6, overclockers everywhere soon found it to be pretty much the most scalable chip yet. Then they found out that it could reach 3.6V without significant performance loss, at which point everyone went nuts. The problem with it was that Winbond left the memory chip IC market shortly after the release of CH-5 and stopped production short on BH-4 which is, as far as anyone knows for sure, a mythical chip with supposed overclocking characteristics that combine the best characteristics of BH-5 and TCCD. The difference between BH-5 and CH-5 is that BH-5 can reach significantly higher clock speeds, rumors have that people have reached well above 320 MHz, however it requires a bit more voltage to do so. And again, simlar to the CH-5, BH-5 can maintain very tight timings at these high clock speeds. BH-5 also has tight timings at stock clock speeds using less voltage than CH-5. For the most hardcore overclockers, there is no better choice than BH-5, if you can find it of course.

Overclocking
Some points about overclocking RAM that may or may not be obvious:

PC 4000 runs at PC 3200. No DDR motherboard will allow you to pop in a 250 MHz RAM stick and have it operating at that speed right off the bat. This is because the stock FSB for all processors right now is at 200 MHz. When you overclock, you can reach that 250 MHz quite often, but before overclocking, that PC 4000 memory you bought will not be running at its optimal speed. That said, the only disadvantage of getting memory rated higher than PC 3200 is the price. Other than that, there is not a “guarantee” that the memory will run with as tight timings as PC 3200 at 200 MHz, but that is not necessarily a disadvantage, depending on how you look at it. The advantages of buying memory rated above PC 3200 are that you will not have to worry about overclocking to 250 MHz or maintaining certain timings at that speed. Memory rated at PC 4000 WILL operate at 250 MHz with the advertised timings. Below that it will work, above that it will work, but there is no guarantee about the timings. PC 3200 (if it is enthusiast RAM) usually will operate at 250 MHz, but with no guarantee on the timings (dependent on RAM chip). Below that it will work, above that it might work, but the only guarantee on the timings is on the low end.

Recommendations
What would this article be without recommendations? Here are the actual products that carry the aforementioned chips:

Samsung TCCD
G.SKILL Extreme Series PC 3200 2 x 512MB
Price: $151.82 (USD)
Stock Voltage: 2.7
Timings: 2-2-2-5

Micron 5B-G
Crucial Ballistix PC 3200 2 x 512MB
Price: $119.00
Stock Voltage: 2.8
Timings: 2-2-2-6

Micron 5B-D
Crucial Ballistix Tracer PC 4000 512MB
Price: $86.00
Stock Voltage: 2.8
Timings: 2.5-4-4-8

Winbond UTT CH-5
Mushkin Redline PC 4000 512MB
Price: $121.95
Stock Voltage: 3.3
Timings: 2-2-2-6

Winbond UTT BH-5
G.SKILL F1 PC 3200 2 x 512MB
Price: $139.50
Stock Voltage: 2.7
Timings: 2-2-2-5


Credits
Micron - Micron Technology Inc.
Samsung - SAMSUNG Semiconductor
G.Skill - DDR Series
Crucial - Crucial BallistiX
Mushkin - XP4000