RAM

Probably won't work at all unless you drop the FSB down to 200MHz, which would seriously slow you down.

DDR and DDR2 are not compatible. Even if they were, your RAM speed is only as fast as the slowest memory module.

There is PC3200 DDR2, and you wont have a problem using both the PC6400 and the PC3200 together, the only thing is that you will not be using a feature called Dual Channel anymore. Dual Channel gives you a 10% speed boost over not using it. You need matching sets of memory to use Dual Channel. Also the system will default to the speed of the slowest stick of ram. With 3gb of ram you may not even notice a difference. Try it and see what happens. You may need to check the voltage and cas of the older stick, it needs to be at least the same voltage. Some motherboards let you tweak the voltage settings, just don't get to aggressive, you might blow your rig up. DDR2 memory is very cheap nowadays, so maybe it's better to buy two 1 or 2 gig sticks for your 'puter to keep it running fast. Hope this helps, Mike.

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Will it be using Dual Channel even though I only have a 2 gig stick of RAM in at the moment?

You need two or four sticks to run in Dual Channel. If you add the other 1gb stick, the computer might slow down.

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It can support Dual Channel, but it only supports up to 800MHz RAM (PC5300?). I don't know if the fsb has any part in this - that is 2000MHz.

Well I think you've got an extra zero in that front side bus. The front side bus speed is set to match the CPU .

The memory speed is determined by using a multiplier for example a 200 MHz fsb will use a multiplier factor of 2 to utilize 400 MHz memory, Wikipedia does a much better job of explaining this than I can.

Back to your machine, PC 5300 is rated at 667 MHz again for the example the multiplier factor for this memory would be approximately 1.67 = 334 memory frequency times 2 for the double data rate and we end up with 668 megahertz, well I told you it was a approximately.

In your particular case if you're using PC2 6400 which has an effective bus speed of 800 MHz it is not running at full speed. The memory can only run as fast as the board will allow.

That's no problem, you can always put faster memory in, however it will still be limited by the motherboards capability.

In your case if I were attempting to use the dual Channel capabilities and wanted to add memory I would replace it with PC2 6400. My reasoning is there is very little price difference, and if you get a newer machine that can handle the faster memory you can always use the memory to upgrade that machine.

Next all memory is not created equal. The memory timings are more aggressive on the premium or high performance memory, so memory running at the same speed can have different data transfer rates, again nowadays there is very little price difference between premium and economy memory so I would get good memory.

The only caveat to this is when purchasing memory. Some of the high-performance memory requires a higher voltage setting, all motherboards may not be capable of doing this, so make sure the memory will run at the standard voltage which is 1.8V.

Just in case it comes up there may be a update for your bias that would allow you to use faster memory, if you haven't done this before I would not attempt it. If something goes wrong you basically killed the motherboard.

http://en.wikipedia.org/wiki/Front-side_bus

It looks to me that you have an AMD processor. When you use Dual Channel, the total clock speed on some motherboards tends to slow down. My machine uses 1066Mhz ram, but when in Dual Channel mode it runs at 800Mhz. My CPU FSB is 2000Mhz because its AMD. Even thou the memory clock speed slows, the gain using Dual Channel is greater then the loss. When I build a computer to sell, I rather install two 1 or 2 gig sticks rather that 1 two or four gig sticks. The performance gain is greater the the Dual Channel that not using it. For your rig I recommend you buy a 2 gig stick to match the one already installed. Just make sure the CAS is the same and it should run in Dual Channel mode. I hope this helped, Mike.

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all will simply run at 200mhz if you had 400mhz + 200mhz inserted

As all memory sees the same clock, the clock would need to run at the slowest speed.

The following is not an answe to your question, but is relevant in case someone extrapolates the answer incorrectly:
It is possible to use memory of similar type (same clock speed and DDR number) that have different delays (CAS ratings) from each other without penalising the faster memory. However, the arrangement of the memory needs to be controlled so that the two types of memory are not addressed simultaneously.

Thanks everyone!

Check with a computer store they can tell you if it will work if you tell them the specs of your computer.