While DDR2 and DDR3 RAM Dual Inline Memory Modules (DIMMs) are physically the same size, and contain the same number of little gold teeth (240), each class of memory is keyed differently and so neither is interchangeable with the other socket.

DDR3 memory modules like the Patriot PDC32G1866LLK memory will not work in a DDR2 memory socket, even if the motherboard chipset technically supports DDR3 - as is the case with the Intel P35 Express, and some boards based on the Intel X38 chipset.

Conversely, DDR3 memory is not backwards compatible with DDR2 memory slots. Not even if you press really hard or bang it in with a hammer.

One obvious difference between DDR3 memory and its predecessor is that DDR3 operates with less voltage. DDR3 RAM requires 1.5V, while DDR2 demands 1.8V power. Next, unique memory slots prevent DDR3 memory from being installed in a DDR2 memory slot, and vice versa. The memory standards themselves are not compatible, so neither are the sockets.

DDR3 memory is not really expected to take hold until mid-2008, when quad-core processors become mainstream; right now they are still too much on the fringe and consequently DDR3 prices are still very high. It's hard to say where AMD will stand on all of this, its 'K10' microprocessor architecture is not expected to adopt DDR3 RAM until 2009 according to the latest reports PCSTATS has seen.

In any case, as with all new memory standards it's important to state the obvious to minimize confusion from the start.

Note the subtle difference where the DDR3 module is 'keyed' with respect to the DDR2 module. Both memory standards have exactly 240 gold connectors, DDR3 operates at 1.5V, DDR2 at 1.8V. Placed edge to edge, it is easy to see the physical difference between DDR2 and DDR3 modules. Where the module is 'keyed' is what separates the modules from bing installed in the wrong slot.