i have some ram in an old computer (its a 400mhz comp) and would like to put it into a slightly less old 800mhz computer, how can i tell if they are compatible?

(i know that processor speed is irelevent to ram compatibility, but hopefully it will give you an idea to the type of ram used with such a computer)
(also i dont belive it matters, but they are both accutally windows boxes(neither of them are mine ))

Check the with the manufacturer.. I'm guessing and its probably gauranteed that your older computer is using PC100 or PC133 168pin SDRAM.. you can most likely see if your newer or faster computers motherboard takes the same type by comparing the RAM slots.. but to be safe, check with the board manufacterer..

Without any details.. we can't help you.. (model numbers) but I'm guessing you should be able to handle searching online for these manuals or tech specs instead of making us do it for you..

Also, what does this have to do with Linux on a direct technical level? I've reported to move this thread to General, where its more suitable to be asked.

The 400MHz machine is almost certainly going to be using a Front Side Bus (FSB) of 100MHz. This means that the RAM is accessed at this speed, and is designated PC100. RAM that is faster than your current machine's FSB will work (so PC133 can be used in a machine that takes PC100), however you cannot use RAM that is slower (so you can't use PC66 RAM in a machine that takes PC100).

Now, the 800MHz machine is highly unlikely to use a FSB of 100MHz. Much more likely is that it'll use either 133 or even more likely, it'll be a DDR machine.

If your 800MHz machine does indeed have a FSB of 133MHz, and you just happen to have been using PC133 memory in your 400MHz machine, then there should be no reason no to re-use it. If, however, you have PC100 memory in your 400MHz machine, then leave it there - you won't be able to use it. And if you should happen to have a DDR 800MHz machine, then you shouldn't even be able to fit the RAM - they are slightly different sizes and have 'notches' in different places.

www.crucial.com has a nice utility for identifying what types of RAM are compatible with a certain motherboard/computer.