Is RAM Reusable?

This may be a dumb question, but I’ve looked for the answer and found it hard to come by. Now my understanding is that when you have a lower grade RAM (such as SDRAM) and use a higher grade RAM (DDR), it will only run as well as the low grade RAM (they both act like SDRAM). Now, say I want to build a new computer from the ground up, but don’t have the money ATM, but my system could definitley use some more RAM now (I have 256 MB SDRAM I think). I figured a quick shot of 128mb would do the trick, but if I were to get DDR instead of SDRAM, can I then later take this out and put it in a new comp? (I hope this isn’t something I should know; I am not very good with technical stuff; but you gotta learn somewhere!). I figure I’m going to do this with lots of my current hardware (read: video card, old monitor, etc). Was just wondering if this is an acceptable practice for RAM. Oh and while were at it, HD’s?
If so, I can go ahead and get some nice RAM and plug it in now.
Only thing I can think of is it would take up a RAM slot on my new MB (is this correct?) and if it’s a small amount of RAM, might be worthless in the long run (since a couple 512’s might work better, than say a 128+256+512). Eh, I know little on this. Thanks for any help you guys have!