ASUS no longer supports the P3C2000, and in fact, this board was recalled.  That's the reason why you can get it for very cheap.
The P3C2000 supports SDRAM only....the MTH (not the NorthBridge....this is actually an additional chip), is soldered down on the motherboard.
Be aware that there are some very strict SDRAM requirements on this board, concerning sizes and DIMM slot order.
The P3C-E board is straight RDRAM only.  It does not have the MTH onboard, and so is not affected by the MTH issue.
What type of RAM a system uses is not really tied to the processor.  You cannot say that the Pentium III is incompatible with RDRAM.  The issue is that due ot the Pentium III's design, along with an FSB that is only 133MHz, the processor cannot take advantage of the additional bandwidth offered by RDRAM, and so shows no performance increase.  This also applies to DDR SDRAM being used on the Pentium III.