zaphate, generally any two sticks of matched memory will support dual channel - if the motherboard does. In most cases, the installed RAM must be added in pairs of nearly identical RAM.
So if you add a single 2GB module it will not pair properly with an existing 1GB module. In rare cases, you'll end up with 2GBs of dual channel and 1GB of single channel. But generally, one of the dual channel controllers will be disabled and all the RAM will be run single-channel.
It's the pair of memory controllers that makes dual channel work.
If you added 1 1GB module or if you added 3 1GB modules to your system, it would likely enable dual channel operation for all 2GBs or 4GBs of memory. This assumes that the memory is similar in design. Or you could add 2 2GBs modules and remove the single 1GB module. If the 2 work as dual channel, try adding the single and see if it still displays as dual channel. This is unlikely tho.
Anytime you add a faster RAM to a system, it will slow down the speed of the slowest RAM already installed. So you can add 800 DDR2 to your existing 667 and all the RAM will run at 667.
Now, all the above sounds like gloom and doom. Keep in mind however, that if you just add 2GBs to your system - even if dual-channel doesn't work - you'll still see a significant improvement in your system speed. You'll boot faster, programs will load and run faster. Hard drive access and delays in spooling printer files will speed up.
Also, if you're running most versions of Windows XP or Vista, the 32-bit versions, and you increase RAM to 4GBs or above, you'll still be limited to around 3.3GBs of RAM available. This is normal and nothing to be done except upgrade to 64-bit version if needed. But again, adding 4GBs is usually the cheapest option for increasing RAM, even if you toss out a 1GB module and can't use it all.
If you post the brand and full model name of your computer, someone here can suggest some RAM options for you. If you live out of the country and have a preferred source for parts, mention it.