Ya I have 2*256 Dual Channel DDR400 PC3200 Ram in already.
My Mobo says "Dual Channel DDR 400 - Supports Dual Channel DDR400 SDRAM, up to 6.4GB memory bandwidth.
I thought I could only put dual channel RAM in my mobo!
So that means I could just stick one 512RAM stick and it would all add up to 1GB of ram?
Yes, but I think you need to spend some time reading up on the particulars of RAM.
Dual-Channel just means that if you have two matched sticks of RAM, in dual-channel slots, then the mobo will treat them like they're one stick of RAM. This usually lowers the latency of the RAM, thereby increasing the speed, but not always true. This is also proportional to the quality of the RAm you buy, and the size of the sticks of RAM. Two 512mb DC RAM sticks aren't going to show as much of an improvement, as the chips are bigger and their latencies tend to be larger.
Also, most mobo's only support 2 slots in DC, even though they may have 4 slots for RAM. You'll need to read your mobo to see how the RAM array is laid out and what configurations of RAM are supported.
You also need to make sure that the rest of your computer is fit for the RAM you bought. Check the EC and buffer type, and make sure your processor can deal with DC RAM (as some, rare though, have had problems supporting DC).
It's always good to read-up a bunch before you dive into something new. That way you can get the best help.