4 sticks of ram come with a performance penalty on all current A64 CPU's, but this doesn't mean you can't overclock well. The limitation is with the onboard memory controller on the A64 chips. With four sticks you are forced to use a 2T command rate and usually have to run ram at DDR333 as opposed to DDR400. To get your maximum overclock you may even have to drop the memory divider to the DDR266 or DDR200 setting, depending on how high you take HTT.
If you don't have a specific need for 2gb of ram, I would recommend running 2x512 as the vast majority of desktop apps and games will not take advantage of the extra memory and will actually run slower with 2gb. But if you do heavy video editing/encoding, CAD applications, Graphics design, Software development etc... and need the extra ram, Just use a memory divider that will ensure that ram isn't the limiting factor in your overclock. CPU speed is much more important than ram speed on A64's