My understanding of memory is that two matched sticks running in the proper slots gives you dual channel mode and most motherboards will only support it in two slots.
Issues can also arise if you fill all memory slots and the memory is not identical which naturally if we're talking 1GB sticks and 512MB sticks it isn't identical.
I don't know what you are using for an operating system but if it's XP then 2GB is sufficient anyway.
To make a completely accurate assessment a person would have to know your motherboard,
RAM specifications for all
RAM being considered and operating system used.
All you ever need to know about Dual Channel Memory......
http://en.wikipedia.org/wiki/Dual-channel_architecture