steal it! haha, i'm in the same boat as you are. RDRAM is so expensive, believe it or not its almost cheaper to spend a little more and get a new motherboard and new ram. that's what i'm doing once i save enough money. it gets cheaper as you can get about $100-$120 for a used stick 512 on ebay.
if you want to find the cheapest place go to fatwallet.com, go to the hot deals section and search for "rdram" and you should get a whole thread dedicated to it. good luck