I am a bit unclear, but this is what I think you are saying...
You have 2X512 of Corsair XMS4000 TwinX
They are running at PC3700. In other words, they are overclocked to around 235MHz from the stock 200MHz(PC3200) system speed, but underclocked from the memory's rated PC4000(250MHz) speed
You want to add: Corsair 1GB DDR XMS4400C25PT TwinX (2x512MB) CAS2.5
So you will have 4X512=2GB total
I am 99% sure that they would work together, but cannot say for 100% sure. Maybe you could ask here
. That is also a good place to look around and see what others are doing. You may want to consider buying them from a place that will be easy to return them if they do not. The 4400 are a bit overkill as the extra bandwidth is wasted due to the 4000 that you currently have. You may be better off with another pair of the 4000 stuff. What ever is cheaper
What are the timings of your current pair? I would guess at least 2.5-3-3-7, if not better(2-3-2-6?) What Vdimm are you using? Let us know how they match up and at what timings you can get both pairs to run at. For fun, you could fool with the new 4400 pair alone