For the "What is the difference question" I would like you to learn from what has already been posted in other threads.
So please use the "Search Boards" on the left of the page.
If you still have questions on the structure or function of ECC, Parity, Buffered, etc. forms of memory then I and others will be happy to assist you.
As for the speed question...
As long as you are going with a motherboard build around the RIMM 2100 then I would suggest the PC1066 over PC800.
PC1066 has the effective bandwidth of 2 of the four data paths for the FSB. So each channel will use 2 and thus the two channels together equal the total for the FSB. PC1066 is much faster. It can interact with the 533MHz FSB on a 1:1 reatio which is what you want. So it is better in reguards to bandwidth and speed.
However if budget is your main focus...then you might want to get som PC800 and overclock from the original 400MHz signal up to the 533MHz signal. Samsung would be good for this.
I am sure others have similar ideas for you.
If you have any other questions about RDRAM or about ECC, etc. just post them here. But please do a search first for the ECC question.
<b>"If I melt dry ice in a bathtub, can I take a bath without getting wet?" - Steven Wright</b>