Yes the Dual Channel does theoretically doubles the bandwidth. You see the processor having a 800MHz FSB has a bandwidth of 6.4 GB/s. And the DDR in single channel does not give the same amount of bandwidth, and hence proves to be a bottleneck in the processors performance. Its like having a Ferrari which cannot go more than 150 kmph
. But when used in the Dual Channel DDR400 mode it gives 6.4 GB/s Bandwidth, and lets the processor work in full power. Means the Ferrari goin in full power
. With dual-channel memory, the memory controller is able to move double the peak amount of data that it could normally move with single-channel memory. This is theoretical though, practically though the performance gain is about 15-25% which too depends on the applications, and helps for memory demanding applications such as 3D graphics in advanced games, 3D modeling, high resolution video editing and playback in addition to other multi application and multi tasking applications. But for the dual channel to work you need to ensure you must use identically paired memory modules in DIMM sockets 0 of channel A and B. Means that 1 stick/module in socket 0 of Channel A and Channel B each. And the below mentioned points:
1. Both modules are the same capacity (e.g. both are 256MB, or 512MB)
2. Both modules are the same speed (e.g. both are PC2700 or PC3200)
3. Both have the same number of chips and module sides (e.g. both have the same number of chips on the module, and both are either single-sided or double-sided).
So in a nutshell, I suggest you get the 2 * 256 and use them in dual channel. Much better than the single 512 module.