Ram problem?

Hello
I have a DFI LANPART SLI D,
Using 3 sticks of ram,
2 x 256 MB Dual Channel:
http://cgi.ebay.ca/ws/eBayISAPI.dl [...] :IT&ih=017
1GB Single CHannel:
http://www.crucial.com/store/parts [...] L12864L503
I first purchased the 1 GB stick, as I got a good deal for it.
I was experiencing lag playing at higher settings with a x800 that was given to me. Knowing the person gave this to me played 2142 with everything on at 1024x768 smoothly, it was troubling me. So I was browsing around on ebay one day and saw the dual channel sticks for sale , I bid 10 dollars and won.
The ram is by the same company, same speed of PC4000, and slightly different timings.
ON the DFI board, the yellow slots are meant for dual channel and the orange ones are preferred for 2T timing memory according to the instructions provided.
When I tried using the dual channel ram in the yellow slots and put the 1 GB in the slower slot , the computer would not start. When I reversed this, it would then boot up, and only when the ram is set to run at 2T
What I dont understand is why? I played around when it was only 1 stick and I was able to put at the same speed as the dual channel's and even bettter timings at that.
So right now I'm not sure what to do, I am getting almost no gain using these sticks, might even be a loss of performance. I am considering just putting the sticks into another computer in the house and buying another stick of 1 GB. Is there any guide on mix matching ram? Any hlep is appreciated,
Thanks.