I have an Asus 7N8X-E motherboard which has dual channel memory. My understanding of this is that two of the memory modules can be acces at once. Am I confused hare? What happens if I stick another module in the third (and last open) slot. Does this impact memory access?

Dual-channel DDR means different things on the NForce2 chipsets vs Intel's P4 chipsets.

With Intel's P4 chipsets, dual-channel means access to both sticks of RAM at once (one per channel, so sticks 1 & 4 can be used at once and not just 1 & 3 and 2 & 4) because the memory controller is 128 bits wide. Being 128 bits wide, tho, has its downsides. First of all, using 1 or 3 sticks of RAM disables dual-channel mode, and the memory controller goes back to single-channel 64-bit mode. Secondly, a dirty little secret of the i865G/GV chipset is that using onboard video automatically disables dual-channel mode (just when you need it the most!). Thirdly, the RAM slots are pre-arranged into channels, so it's possible for a newbie to accidentally install RAM into slots 1 & 2 (on the same channel) and not get dual-channel mode.

NVidia's NForce2 memory controller is actually 64 bits wide with oscillating abilities, meaning it sends and receives data to only one stick of RAM at a time, but it can move between sticks very fast. This way isn't as efficient but does give a performance boost because the memory controller can give a command to one stick and then to another stick while the first stick is still responding (remember, RAM has lots of latencies). The performance boost on a SocketA system is disappointing to say the least, but this is mainly because SocketA CPUs don't have the bandwidth to really benefit from dual-channel mode. The upside to the NForce2 chipset is that it's very robust. First of all, using dual-channel mode with onboard video gives a significant performance boost to budget systems because the CPU can use one stick of RAM while the onboard video buffer uses the other. Secondly, installing 3 sticks doesn't destroy dual-channel mode like on Intel chipsets, it just makes the last stick operate in single-channel mode while the first two enjoy dual-channel.

Ok, probably more info than you needed but I tend to ramble on. The simple answer to your question is go ahead and add a third stick of RAM. You won't hurt your RAM performance at all.