Dual Channel is not Double the Bandwidth:

Since high-speed DDR brings some problems, it's only natural to look for ways of increasing the bandwidth of lower-speed DDR. The latest solution is to adopt a dual-channel bus to system memory, thereby doubling the potential bandwidth without having to increase memory clock speeds much higher than normal. This is a good solution to the bandwidth problem, since vendors can use JEDEC-authorized specifications yet still achieve higher-than-DDR400 data transfers. If you look at benchmarks, a dual-channel configuration is about 1-5 percent faster on average. Some specific system benchmarks may go as high as 15 percent.

With an Althon XP processor you will not gain any advantage from dual-channel DDR400 due to the limited bandwidth of the Athlon XP's front side bus (3200MB/second), however a memory controller can never operate at 100 percent efficiency, so you will actually get a memory bandwidth that is noticably lower than 3200MB/second for a single-channel DDR400 configuration. Dual-channel will eliminate most of the loss caused by the memory controllers inefficiency, so that's why you do get a slightly higher memory bandwidth in a dual channel configuration.

Pentium 4 CPU's, however, fare much better with the dual channel memory because of their higher bus speeds (800mhz) and therefore higher CPU bandwidth.

Memory bandwidth is not everything, actual performance depends on the individual application you're running.

On average, an Athlon XP performs equal to a Pentium 4 with a corresponding model number, the difference comes into play with the XP 3000+ and 3200+ models when compared to 3.0GHz P4 "C" with 800MHz front side bus and subsequent models in conjunction with Intel's i865/i875 dual-channel platforms this is where the P4 shows its ability to produce high benchmarks.

Intel's Latest offerings and Bandwidth:

Motherboards based on the Intel 915G Express chipset, designed for the Intel Pentium 4 processor supporting Hyper-Threading (HT) Technology in the LGA775 package provide flexible memory support for both dual channel DDR2/533 or DDR2/400 memory, which can deliver up to 8.5 GB/s bandwidth, and dual channel DDR memory allows for a range of memory configurations.

Intels new chipsets support Intel's 800MHz front side bus processors with dual-channel DDR2/533 or DDR2/400 memory configurations up to 4GB.

Five new 90nm process Pentium 4 CPUs, each with an 800MHz front side bus and 1MB of Level 2 cache, use Intel's new model numbering system:

Intel Pentium 4 520 (2.8GHz)
Intel Pentium 4 530 (3.0GHz)
Intel Pentium 4 540 (3.2GHz)
Intel Pentium 4 550 (3.4GHz)
Intel Pentium 4 560 (3.6GHz)

The 0.13-micron-process, 3.4GHz Pentium 4 Extreme Edition with 512K of L2 and 2MB of L3 cache will also move over to the LGA775 socket.

AMD's Latest offerings and Bandwidth:

AMD Athlon 64 FX-53 (939-pin)
AMD Athlon 64 3800+ (939-pin)
AMD Athlon 64 3700+ (754-pin)
AMD Athlon 64 3500+ (939-pin)

Socket 754 CPUs have 64-bit integrated DDR memory controller with up to 6.4GB/sec memory bandwidth.
 
Socket 939 CPUs have the  memory controller still on the die, but the socket 939 processors do not require you to use registered memory.  Standard unbuffered modules will have a performance gain in the memory bandwidth along with lower memory cost. The HyperTransport link has been raised from 800MHz to 1GB thus providing an increase from 6.4GB/s to 8GB/s on the HyperTransport bandwidth.

We hope we have provided you with some basic understanding of how memory bandwidth works and how it's related to the overall system performance.