* http://www.experts-exchange.com/Hardware/Components/Memory/Q_23336426.html

It's always best with unbuffered memory to only install 2 modules => the electrical load of 4 modules notably distorts the waveform of the address and data signals and can have a real impact on reliability.   Some BIOS programs will reduce the speed when 4 modules are installed; some will switch to single channel mode; etc. to try and improve reliability.   I'm convinced a lot of spurious errors and unexplained crashes are simply due to memory "glitches" caused by waveform distortion.

Switching to 2 x 2gb modules will certainly improve the reliability and stability of your memory subsystem;  but the change from PC-8500 to PC-6400 will also slow it down a bit.   The best alternative would be to get 2 x 2gb PC-8500 modules :-)

The key to how much load you're putting on the address & data buses is how many actual memory chips are on the modules.   Single-sided modules have 8; double-sided modules have 16.   So four single-sided modules present the same load as two double-sided modules.   But with 1GB (& above) modules you're most likely using all double-sided modules ... so I'd definitely limit the load to two modules if you want the most reliable memory subsystem.

... one of these days the desktop motherboard makers will offer motherboards that use buffered RAM modules (like server boards) => then you can use as much memory as you want (the load on the address & data bus drivers than becomes one load per installed module instead of one load per memory chip).