Would this just mean 2 x 512mb DDR of any make
Nope has to be same size, same specs, same manufacturer....
I honestly couldn't tell you if the 3500A would work with 3500 to obtain dual channel....it could have barely enough difference internally to cause it to not run...but there's only one way to find out.
Sometimes computers can be fickle and not run DC when it should and sometimes you can get lucky and it'll run DC when it shouldn't..
Basically it seems like you really have no choice in the matter.
You're just going to have to buy it and see if it works...
Besides DC isn't really THAT important...it's anywhere from 5%-20% gain so I doubt you'll notice a huge decrease in speed without having DC