don't you mean 2 DIMMs or 4? Anywho..
Certain Intel boards (such as the i875p chipset) perform better with 4 double-sided modules running at DDR-400.
This is only applicable if your system IS running at DDR-400 with the current memory. If, for some reason, your system runs the four modules at DDR-333 speeds then, according to Intel, you are better off with 2 modules in terms of both performance and stability (assuming same total memory size for both cases).
2x1024MB in my opinion is better. Besides, the performance difference between 2x1024MB and 4x512MB is negligible. However, you'll have more problems in general running four modules together.
<font color=blue>******
<font color=green>"My Memory is not working today"
<font color=blue>AIM SoCaliCrowley