Helping people with computers... one answer at a time.
Depending on your computer you may be able to mix and match RAM chips of differing sizes. Or you may not.
I just purchased 1 gigabyte of RAM for my computer and it works great. What didn't work was trying to use my previous 2 256MB ram sticks in combination with my new 1GB Stick. Is there any way to make this combo work? Perhaps a specific order to install the RAM?
The short answer is: maybe.
On some computers you can, and on others you cannot. On some you can, if as you suggest, you do it "the right way". So how do you tell?
It really all depends on how the computer's motherboard is designed. The definitive answer comes from your computer manufacturer. My first step is always to visit the support web site for that specific computer and see what's available, and how memory can be configured.
Another good resource is Crucial.com. I've purchased memory from them a time or two, and they have a fairly helpful compatibility identifier which can step you through the process of selecting the proper memory for your computer.
The "problem" is simply that different computer motherboards can accept memory in different, but specific, combinations. Some examples:
Some require that all memory sticks in the computer be identical. So if you have four slots, you can have 4x256meg, or 4x1gig, but you can't mix the 256meg and 1gig sticks.
Some allow you to mix, but you must mix in pairs. Meaning you might be able to have 2x1gig and 2x256meg.
Some require that if you mix, you must do so in a certain sequence. So you might be able to put 1gig in slot 1, and 256meg in slot 2, but not the other way around.
Many require that if you mix (according to whatever other rules there might be), the memory sticks share certain other characteristics, such as speed or others.
As you can see it's not a simple yes you can, or no you cannot. It really does depend on the machine you're dealing with.