|"If the computer runs at the slowest speed of the smallest RAM stick you have in combination with your BIGGER sticks, then removing the smallest stick(s) should make your computer run faster because it'll then be running at the speed of the bigger stick. And I don't think that happened."|
That is sort of true, but I think you have some misconceptions here. Assuming they are the same type of RAM (e.g. PC2700, PC2100), all sticks will run at the same speed regardless of their size. A 512MB stick of PC2100 runs at the same speed as a 1GB stick of PC2100, by definition. RAM speed is determined by the type, not the size.
You originally went from 512MB to 3GB, so of course you're going to notice a big apparent difference in speed from that. This is due to the increase in amount of installed ram, and has nothing to do with the size of any individual stick. But you also say you notice a speed increase when removing the 256MB stick. Compare the 1GB sticks to the 256MB stick. Are they the same type? You can find some information about RAM types here to help you identify what you have. I suspect the 256MB stick is a slower type than the larger sticks, in which case you're probably right...the system is downclocking to the speed of the slowest installed stick. Which, again, has nothing to do with the SIZE of the stick and everything to do with the TYPE.
Now, if all installed sticks are the same type the system may "feel" faster or slower when more or less RAM is installed. This is not due to how fast the RAM runs, but rather how the system actually works when different amounts of RAM are installed. With 512MB installed, the system does a lot more paging than when more RAM is installed. So even though both amounts of RAM function at the same speed, the overall system "feels" slower. Again, this is due to the amount of RAM, not the type.
Your method of testing speed, is inherently a measure of how the system "feels". When you multitask by running several programs, the system must swap a lot when there is less RAM, and will "feel" slower as a result. This is what you observed, right?
How to ask a question.