Assuming this is your motherboard,
and these are your specs
Your motherboard supports pc3200, but has pc2700 installed (it supports pc2100, pc2700 and pc3200). You need standard ddr ram (dual channel has nothing to do with the type of ram). Dual channel is the process if taking two memory sticks, and basically reading and writing to both at the same time to double speed.
Most mother boards are capable of running dual channel or single channel configurations. If you run single channel, you can run diffrent size sticks and even diffrent speed sticks (pc2100/pc2700/pc3200 are the speeds) but you take a performance hit. Dual channel gives a performance increase but requires two identical memory sticks (same brand, model, speed etc). Memory sticks are even often sold in matched pairs from the same lot to help insure they are identical as memory can very slightlly from lot to lot. There are exceptions to the rule that they have to be identical but there are no guarentees.
I would try to find out what diffrent memory configurations your motherboard can run. Some operations run beter with more memory even if the speed is slower, others will benifit more from faster ram and or dual channel operations. It might be easiest to just pull the pc2700 out and get a matched pair of pc3200 memory (you need to verify that is in fact your motherboard to ensure that it is pc3200 compatible).
You would still need to find out what memory configurations your motherboard supports as you generally need to install the ram in certain slots to run dual channel. You could of course buy 512 or 1 gig and run it with the 512 you already have but again you need to know what memory configurations it will run.
From what I can tell searching quick, your motherboard has 4 memory slots and can only run dual channel, so memory must be installed in pairs.
I am not sure if your motherboard would support running the two 256's that you have now in two slots, and two new diffrent speed cards in the other two or not, but worst case senerio, you can run the two I linked to above (by themselves without your current memory for sure).
I hope that helps but I would look for a little more info to make sure of everything (but hopefully that will help you understand it beter).