I own a Dell Dimension 8250 and have just put in some new RDRAM.  The
original RAM is ECC (Error Code Correcting), but the new RDRAM is
non-ECC. The computer runs faster now with the new RAM, but only
registers half the total memory in system (320/756MB). The original
RAM is 128MBx2 (256MB) and the new RAM (non-ECC) is 256x2 (512MB).
There are four slots on my motherboard, all full, and the computer
does not boot when I put the new RAM where the original RAM is. How do
I make the computer recognize all of the RDRAM?

It may have been that the RDRAM you bought is not compatible with your
motherboard. http://www.crucial.com has a system scanner that scans
what type of computer you have (eg make, model, manufacturer, etc.)
and it will recommend a several types of RAM, SDRAM, DDR RAM, and
RDRAM. Then, use the make and model to scour for a better price than
crucial. Hope this helps.

I have a Dimension 8250 and I upgraded the ram from 512 to 1 gb. 
Since the price of RDRAM is so expensive, I decided to go with the
same brand that was originally in the system, which happened to be
SAMSUNG RDRAM 1066.

Are you running XP SP2?