The main advantage of DDR-II over DDR is its ability to work at higher clock speeds. This is achieved through an increased number of buffers, an improved prefetch, reduced electrical requirements, improved packaging and on-die termination. However, it also sacrifices latency to achieve this.
The first commercial product to use the technology was the GeForce FX 5800 series, but it caused severe isssues with heat dissipation, necessitating large heatsink/fan arrangements on the 5800 and Quadro FX 2000, and the infamous "Flow FX" cooling system used on the 5800 Ultra. The 5900 series reverted to DDR SDRAM, as did the 5950, but nVidia’s newest mainstream card, the 5700, uses DDR-II clocked at 900MHz (compared to 800MHz on the 5800, and 1GHz on the 5800 Ultra).
ATI's Radeon 9800 with 256MB memory (not the 128MB version) also used DDR-II, but this was because DDR-II requires fewer pins than DDR. The Radeon 9800 256MB only runs its memory at 20MHz faster than the 128MB version, primarily to counter the performance loss caused by higher latency and the increased number of chips. It's thought that the DDR-II used on the 9800 256MB is actually memory that was supposed to be used on the GeForce FX 5800 series, but ended up being unused after nVidia decided to halt production of the 5800 line. The newest ATI chip, the 9800XT reverted to DDR, and ATI is expected to bypass DDR-II in it's future products, instead using GDDR-3.
The speed at which DDR-II will be introduced is currently unknown. It will not be 400MHz, because at that speed it would perform worse than DDR400 because of the increased latency. Some manufacturers have introduced DDR modules capable of 533MHz operation. While these modules would outperform any possible DDR-II 533 modules, these "DDR533" modules run with their voltage somewhat outside of the capabilities of most motherboards. Intel's "Grantsdale" chipset (due for launch in 2004) is currently expected to run dual-channel DDR-II 533, to match up with the Pentium 4 "Prescott" being boosted to a 1.066GHz front-side bus.
The heat issues that Geforce FX 5800 (and to a lesser degree the Radeon 9800 Pro 256MB) suffered from should not be as much of a problem, since JEDEC-approved DDR-II will most likely be produced on a 0.09 micron process (as opposed to 0.11 or 0.13 microns on most current DDR-II modules), and will run at lower clock speeds. Moreover, the lack of heat problems on the GeForce FX 5700 suggests that the thermal problems with the 5800 series may have been because of the combined heat output of the memory and the GPU.
The main competing technologies are as follows: