Doing a quick search on ebay Rambus RDRAM reveals quite a few auctions that might be within your price range.
Otherwise if you decide to go to another motherboard/cpu you can use your Dell components on it. Just like was previously mentioned you need to use Dell's drivers for the sound and video, etc. They tend to buy modified versions of the cards that typically don't work right without the Dell driver. All that is easily obtained from Dell's support website.
If you do go another computer route, you might be able to sell the remaining pieces on ebay to soften the blow.
