<< It uses DDR, which is so much cheaper than RDRAM >>
From pricewatch:
256Megs of PC2100: Generic, $50. Crucial, $70
256Megs of PC2700 (Needed on P4S333 or it's not worth getting over 850): $71
256Megs of PC800: Generic $66, Samsung $75
So, as you can see, there is next to no difference between PC2700 and PC800. I really would suggesting taking Asus P4T-E because future Northwood's will use a 533fsb and will benefit much more from it on a RDRAM platform.